UC-NRLF 


^B    273    bT4 

ELEMENTS 

OF 


APPLIED  ELECTRICITY 


BY 

H.  H.  BLISS 

State  Supervisor  of  Trade  and  Industrial  Education  for  Nevada, 

Formerly  in  charge  of  Extension  Engineering  Courses 

for  the  University  of  California 


ii 


•i  \-:A 


PUBLISHED  BY 

JOURNAL  OF  ELECTRICITY 

SAN  FRANCISCO.  CALIFORNIA 

1920 


Digitized  by  the  Internet  Archive 

in  2007  with  funding  from 

IVIicrosoft  Corporation 


http://www.archive.org/details/elementsofapplieOOblisrich 


ELEMENTS 

OF 

APPLIED  ELECTRICITY 


H.  H.  BLISS 


State  Supervisor  of  Trade  and  ntdrCStrml  Education  for  Nevada . 

Formerly  in  charge  of  Extension  Engineering  Courses 

for  the  University  of  California 


FIRST  EDITION 


Copyright.  1920.  by  Journal  of  Electricity 


PUBLISHED  BY 

JOURNAL  OF  ELECTRICITY 
SAN  FRANCISCO.  CALIFORNIA 

1920 


PREFACE 


What  do  you  know  about  electricity?  Can  you  explain 
simple  circuits,  losses,  power  and  efficiency,  wiring  calcula- 
tions, how  generators  and  motors  are  installed,  how  they 
work,  what  efficiency  means  and  how  to  calculate  it,  and  how 
current  for  electric  lighting  and  heating  is  estimated? 

"Know  the  fundamentals"  is  the  cry  of  the  hour.  Here 
is  a  series  of  discussion  which  has  appeared  in  the  columms 
of  the  Journal  of  Electricity  in  cooperation  with  the  Extension 
Division  of  the  University  of  California  on  the  all-important 
subject  of  elementary  laws  of  electricity.  The  forwarding 
of  this  movement  is  a  matter  that  strongly  appeals  to  every 
member  of  the  electrical  industry  —  manufacturers,  jobbers, 
central  station  men,  electrical  contractors  and  dealers — and 
has  received  the  heartiest  endorsement  of  the  electrical  indus- 
try from  all  quarters.  These  discussions  which  appeared  in 
the  columns  of  the  Journal  of  Electricity  during  the  year  of 
1919-1920  under  the  endorsement  of  the  California  Electrical 
Cooperative  Campaign,  an  organization  composed  of  all  mem- 
bers of  the  electrical  industry,  have  received  wide  and  em- 
phatic  endorsement. 

The  author,  Mr.  H.  H.  Bliss,  for  a  number  of  years 
was  head  of  the  technical  instruction  of  the  Extension  Divis- 
ion of  the  University  of  California,  and  while  occupying  that 
position  gave  this  course  through  the  University  Extension  in 
cooperation  with  the  Journal  of  Electricity.  The  course 
proved  unusually  successful,  and  aroused  interest  throughout 
the  West  in  the  study  of  fundamentals.  It  is  with  this  same 
hope  that  this  group  of  papers  may  prove  of  increasing  help- 
fulness that  the  Journal  of  Electricity  has  compiled  these 
pages  into  book  form  in  order  that  a  permanent  record  may 
be  had  with  these  papers  in  one  volume  so  that  the  biggest 
and  most  intensified  use  of  this  valuable  collection  may  be 
offered  to  that  ever  growing  group  of  young  and  enthusiastic 
as  well  as  ambitious  men  in  our  industry  who  wish  to  forward 
themselves  to  greater  remuneration  from  their  employers  and 
to  greater  usefulness  in  their  chosen  profession. 

ROBERT  SIBLEY,  Editor, 

Journal  of  Electricity. 


■;1  -'  .^oiri 


^>TT 


TABLE  OF  CONTENTS 


Chapter  Page 

IV  Ohm's  Law  and  the  Electric  Circuit 1 

IL  Series  and  Multiple  Circuits 8 

III.  Power — Losses — Efficiency 15 

IV.  Electromagnets — Transformation  of  Energy 21 

V.  Wire  Calculations  28 

VL  The  Generator  35 

VII.  Armature  and  Field  Windings 42 

VIII.  Losses  and  Reactions  in  D.C.   Generators 49 

IX.  Electrolysis '.....  55 

X.  Electric  Motors 63 

XL  Motor   Characteristics 71 

XIL  Electric  Meters 78 

XIII.  Lamps   and   Illuminations 86 

XIV.  Induction — Transformers — Interpoles 94 


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ELEMENTS 

OF 

APPLIED  ELECTRICITY 

I 

OHM'S   LAW  AND  THE   ELECTRIC   CIRCUIT 

Our  discussion  of  electrical  principles  and  prac- 
tice begins  with  the  consideration  of  Ohm's  Law, 
which  is  the  basis  of  all  quantita- 
tive knowledge  of  circuits  and  ma- 
chines. Its  fundamental  character 
is  recognized  in  the  industry,  and 
the  National  Electric  Light  Associ- 
ation has  adopted  *  for  its  official 
emblem  the  Ohm's  Law  formula 
"C  =  E/R,"  which  appears  upon  all  the  stationery 
and  official  documents  of  this  nation-wide  organi- 
zation. 

Electric  Currents. — In  order  to  utilize  electric 
energy  it  is  necessary  to  connect  the  source  of  the 
current,  such  as  a  battery  or  generator,  to  other 
apparatus,  such  as  motors,  heaters,  or  lamps.  There 
must  be  a  continuous  path  for  the  current  from  the 
source  to  the  point  of  use  and  back  again  to  the 
source.  As  soon  as  this  circuit  is  broken  at  any 
point  the  current  stops. 

The  materials  which  can  carry  electricity  are 
called  "conductors."  They  include  all  metals,  both 
when  solid  and  liquified  (as  mercury  or  melted  iron) ; 
carbon;  impure  water;  earth;  moist  woods,  etc. 
Materials  which  stop  the  flow  of  electricity  more  or 
less  completely  are  termed  "insulators.''    These  in- 


2 ':  V^  /  '.        APFIilED     ELECTRICITY 

^dIi^;lrteL^^s^•|)of•cela^^^  marble,  slate,  rubber,  paper, 
cloth*  wax,  ary\^6od','etc.  The  fact  that  any  water, 
except  chemically  pure  distilled  water,  can  carry 
electricity  causes  such  materials  as  wood,  cloth, 
paper,  dirt,  etc.,  to  fall  into  one  class  or  the  other 
according  to  whether  they    are  dry  or  wet.     And 


A  current  of  gas  may 
be  measured  in  cubic 
feet  per  minute,  by 
means  of  this  meter 
and  a  watch;  an  elec- 
tric current  is  more 
easily  measured,  in 
"coulombs  per  second" 
or  "amperes,"  by  means 
of  a  single  instrument, 
the  ammeter.  The^cur- 
rent  in  either  case  must 
go  through  the  meter. 
(See  Fig.  1., 


small  particles  or  veins  of  metal  in  insulating  ma- 
terials sometimes  lead  the  current  to  places  where 
it  is  a  source  of  annoyance  or  danger.  Air  is  gen- 
erally an  insulator,  but  under  certain  circumstances 
it  becomes  a  conductor,  as,  for  example,  in  the  elec- 
tric arc  where  large  currents  flow  for  a  short  dis- 
tance through  air. 

Measuring  Electric  Current.  —  A  current  of 
water  in  a  pipe  or  a  river  can  be  metered  in  various 
ways,  and  the  rate  of  flow  can  be  stated  in  terms  of 
gallons  per  second.  In  a  similar  way  the  rate  of 
flow  of  an  electric  current  can  be  stated  as  so  many 
"coulombs  per  second,"  but  it  is  more  customary  to 
substitute  for  this  phrase  the  single  word  "amperes." 
A  statement  that  "the  current  is  16  amperes"  means 
that  16  coulombs  pass  a  given  point  in  the  wire 
every  second. 


OHM'S     LAW    AND     THE     ELECTRIC     CIRCUIT      3 

Tungsten  lamps  take  currents  ranging  from  .23 
to  .91  amperes  in  the  sizes  commonly  used  (25  to 
100  watts) ;  arc  lamps  take  from  3  to  20  amperes ; 
a  10  horsepower  motor  on  a  250  volt  circuit  will  take 
about  40  amperes. 

To  measure  the  rate  of  flow  in  an  electric  cir- 
cuit we  use  an  instrument  called  an  ''ampere  meter" 
or  "ammeter.''  It  is  inserted  into  the  circuit,  as 
shown  in  Fig.  1,  so  that  the  current  must  go  through 
the  instrument  between  the  source  and  the  load. 
A  needle  shaped  pointer  moving  over  a  scale  gives 
a  reading  of  the  current  in  amperes. 


Fig.  1- — The  current  goes  through  the  ammeter  between  the  battery  and 
the  lamp.  When  the  switch  (S)  is  opened  the  battery  (B)  can  no  longer 
send  current  to  the  lamp  and  the  ammeter  needle  points  to  the  zero  mark. 

Resistance. — ^If  in  the  circuit  of  Fig.  1  we  re- 
place the  lamp  by  one  of  different  candle  power  or  by 
a  piece  of  fine  iron  wire  or  by  an  electric  bell,  we 
shall  find  the  ammeter  giving  an  entirely  different 
reading.  The  battery  tries  equally  hard  to  force 
electricity  through  the  circuit,  but  the  amount  it 
can  send  depends  upon  the  apparatus  through  which 
the  current  must  flow.  We  may  say  that  the  lamps 
differ  in  the  amount  of  "resistance"  they  offer  to  the 
passage  of  electricity.  If  one  takes  three  times  as 
many  amperes  as  a  second,  we  may  say  that  it  has 
one-third  the  resistance  of  the  second. 

Electrical  resistance  is  measured  in  "ohms." 
It  is  thought  of  as  a  sort  of  "electrical  friction," 
like  the  opposition  a  rough  pipe  offers  to  the  flow  of 
water  through  it.  The  resistance  of  a  25  watt  Tung- 
sten lamp  is  about  485  ohms;  that  of  an  electric 


4  APPLIE^D     ELECTRICITY 

iron,  about  25  ohms;  the  resistance  of  a  piece  of 
copper  wire  1/10  inch  in  diameter  and  1000  feet 
long  is  one  ohm. 


When  this  switch  is 
opened  it  stops  the  cur- 
rent in  a  high  tension 
power  line  by  interpos- 
ing an  air  resistance  of 
millions  of  ofhms.  When 
closed,  the  resistance 
of  the  switch  is  practic- 
ally zero.  The  current 
carried  may  amount 
to  300  amperes.  To  pre- 
vent the  escape  of  cur- 
rent, under  the  enor- 
mous pressure  of  110,- 
000  volts,  the  switch 
has  to  be  supported 
upon  these  huge  insu- 
lators. Switches  of  this 
type  are  to  be  used  in 
connection  with  the 
Chicago,  Milwaukee  & 
Pu get  Sound — the  first 
electric  transconti- 
nental railway. 


Pressure. — We  come  now  to  the  consideration 
of  a  third  factor  in  electric  circuits,  namely,  the 
"pressure"  which  forces  the  current  through  the 
wires.  There  is  evidently  something  in  a  battery 
or  an  electric  generator  which  forces  electricity  to 
go  out  at  one  terminal  and  to  come  back  at  the 
other,  just  as  a  pump  sends  water  out  at  one  place 
and  draws  it  in  at  another.  Of  course,  an  open 
switch  or  a  closed  valve  may  block  the  flow,  but  the 
electric  pressure  or  water  pressure  is  still  ready  to 
start  the  current  when  opportunity  is  offered. 

Water  pressure  is  measured  in  pounds  per 
square  inch,  by  means  of  a  pressure  gage.  The  unit 
of  electrical  pressure  is  the  "volt."  The  pressure 
or  "voltage"  in  ordinary  house  circuits  is  about  110 
volts;  a  dry  battery  has  a  pressure  of  about  1.5 
volts;  the  voltage  applied  to  street  car  motors  is 
usually  about  550. 


OHM'S     LAW     AND     THE     ELECTRIC     CIRCUIT      5 


There  should  be  no  confusion  about  the  words 
ampere  and  volt.  The  number  of  amperes  indicates 
the  rate  of  flow,  without  reference  to  the  pressure 
driving  the  current.  Then  "110  volts''  indicates  only 
a  tendency  to  send  current  with  no  reference  to  how 
much,  if  any,  actually  flows.  We  may  have  110  volts 
and  1,  5,  or  500  amperes,  or  no  flow  at  all,  depending 
upon  the  resistance  in  the  circuit. 


V 


1 

M 

1 

Fig.  2. — Voltmeter  Vi  measures  the  electromotive  force  of  the   generator ; 
V2   measures   the   pressure   applied    to    the    motor. 

To  measure  pressure  we  use  a  "voltmeter,"  an 
instrument  which  somewhat  resembles  an  ammeter. 
To  find  what  voltage  is  sending  (or  available  to 
send)  current  through  a  circuit  or  piece  of  appara- 
tus, we  connect  one  terminal  of  the  voltmeter  to  each 
end  of  the  circuit  or  to  each  terminal  of  the  appa- 


Here  is  an  electric  warming  pad — quite  a  companion  on  cold  nights. 
Attached  to  a  110  volt  circuit  it  takes  a  current  of  one-half  an  ampere. 
What  is  its  resistance? 

ratus.  In  Fig.  2  the  voltmeter  marked  ''Y^'  meas- 
ures the  ^'voltage  across  the  motor"  (the  pressure 
tending  to  send  current  through  the  motor),  while 
the  other  voltmeter  measures  the  pressure  the  gen- 


APPLIED     ELECTRICITY 


erator  exerts  to  send  current  through  the  whole  cir- 
cuit. The  two  instruments  need  not  give  equal 
readings. 

Ohm's  Law. — One  volt  is  the  pressure  needed  to 

send  one  ampere  through  one  ohm  resistance.  From 

this  it  follows  that  the  number  of  volts  required  to 

send  a  current  through  any  resistance  is  equal. to 

the  product  of  the  numbers  of  amperes  and  ohms. 

This  statement,  which  is  known  as  Ohm's  Law,  may 

Here  is  a  typical  exam- 
ple of  the  use  of  insu- 
lators and  conductors  in 
long  distance  transmis- 
sion of  electric  power. 
The  famous  crossing  of 
the  lines  of  the  Pacific 
Gas  &  .  Electric  Com- 
p  a  n  y,  at  Carquinez 
Straits  in  California 
was  for  years  the  most 
daring  enterprise  of  its 
kind  and  today  it  ranks 
as  the  second  longest 
span  in  the  world.  Each 
of  the  six  cables  con- 
sists of  19  strands  of 
steel  wire,  making  a 
composite  size  for  each 
cable  of  %-inch  diam- 
eter, with  the  remark- 
able length  of  6200  ft. 
Assuming  that  a  cable 
6200  ft.  long,  equiva- 
lent to  No.  1  copper 
wire,  has  a  resistance 
of  0.77  ohms  and  a 
carrying  capacity  of 
150    amperes,    what    is 

the  voltage  required  to  force  the  current  through  this 
resistance,  according  to  Ohm's  Law? 

be  indicated  by  a  brief  formula :  "Volts  =  amperes 
X  ohms.''  Other  ways  of  writing  it  are:  "Am- 
peres =  volts -f- ohms"  and  "Ohms  =  volts  -^  am- 
peres.''   All  three  formulas  should  be  memorized. 

The  second  of  the  three  formulas  may  be  writ- 
ten :  "Current  =  electromotive  force  -^  resistance." 
Using  initials  instead  of  words,  C  =  E  -^  R.     This  is 


OHM'S    LAW    AND     THE     ELECTRIC     CIRCUIT      7 

the  symbolic  representation  of  the  law  as  used  in 
the  emblem  of  the  N.  E.  L.  A.  shown  at  the  head  of 
the  chapter. 


II 


SERIES  AND  MULTIPLE  CIRCUITS 

Series  Circuits. — Many  electric  circuits  consist 
of  several  different  parts  through  which  the  current 
passes  in  "series."  This  means  that  the  electricity- 
must  go  through  one  part  after  another.  Make  a 
clear  distinction  between  this  arrangement  and  the 
"multiple"  circuit  in  which  the  current  divides  and 
flows  through  several  branches.  Fig.  3  illustrates 
the  first  and  Fig.  4  the  second  type. 

What  is  said  about  these  applies  to  direct  cur- 
rent circuits  and  also  to  those  alternating  current 

aohms 


120  V. 


SLohma 

Fig.  3. — Resistances  in  Series.  In  spite  of  the  varying  resistance  of  the 
various  elements  of  the  circuit,  the  current  passing  through  each  is  the 
same — 3   ajmperes. 

circuits  which  contain  only  simple  resistance  and  no 
electromagnetic  apparatus  or  condensers. 

The  current  leaving  the  generator  in  Fig.  3  goes 
first  through  an  ammeter,  then  through  the  upper 
wire,  then  through  a  lamp,  then  through  a  resistance 
coil,  and  finally  back  to  the  generator  through  the 
lower  wire.  It  is  obvious  that  if  3  coulombs  per 
second  (3  amperes)  pass  through  the  generator  to 
the  ammeter,  the  same  number  must  travel  along 
the  upper  wire  through  the  lamp  and  the  coil  and 


8 


SERIES    AND    MULTIPLE     CIRCUITS  9 

back  to  the  generator  through  the  lower  wire.  If 
any  more  coulombs  passed  out  through  the  ammeter 
than  came  back  through  the  lower  wire  there  would 
be  an  accumulation  of  electricity  somewhere  in  the 
right  hand  part  of  the  circuit ;  if  any  more  returned 
to  the  generator  than  left  it  there  would  be  a  pro- 
duction of  coulombs  somewhere  in  the  right  hand 
part  of  the  circuit.  Both  of  these  alternatives  are 
impossible  with  this  apparatus  and  hence  we  con- 
clude that: 

In  a  series  circuit  the  current  is  the  same 
everywhere. 

If  a  resistance  of  2  ohms  is  in  a  series  of  another 
of  3  ohms,  the  electromotive  force  must  overcome 
5  ohms  in  sending  current  around  the  circuit.  If  we 
apply  20  volts  the  amperes  will  amount  to  20-^-5 
or  4. 

In  a  series  circuit  the  total  ohms  =  the  sum  of 
separate  resistances. 

To  find  the  voltage  across  each  one  of  the  re- 
sistances in  the  previous  example  we  apply  Ohm's 
Law  as  usual :  4X2  =  8  volts  across  one,  and 
4  X  3  =  12  volts  across  the  other.  Across  the  whole 
combination  the  pressure  =:  4  amps.  X  5  ohms  =  20. 
This  result  is  also  found  by  adding  the  two  voltages 
8  and  12. 

Pressure  across  a  series  of  resistances  equals 
sum  of  the  voltages  across  the  separate  resistances. 

In  Fig.  3  a  lamp  of  31  ohms  resistance  is  in 
series  with  a  coil  having  5  ohms.  Current  is  supplied 
from  a  120  volt  generator  through  two  line  wires  of 
2  ohms  each.  What  current  flows  and  what  pressure 
is  used  to  force  this  current  through  the  lamp  ?  The 
total  resistance  is  40  ohms.  Hence  the  current  ^=3 
amperes.  To  drive  3  amperes  through  31  ohms  re- 
quires 3  X  31  or  93  volts,  which  is  the  pressure 
across  the  lamp. 

Voltage  Drop. — In  this  example  we  find  the 
pressure  across  the  lamp  considerably  lower  than 
that  supplied  by  the  generator.     There  has  been  a 


SERIES    AND    MULTIPLE    CIRCUITS 


11 


drop  in  the  voltage  from  120  to  93,  or  27  volts.  This 
may  also  be  calculated  by  Ohm's  Law.  The  resist- 
ance of  the  circuit  between  the  generator  and  the 


.Oohrm 


Fig.  4. — Resistances  in  Multiple.  The  current  divides  between  the  lamp 
and  the  resistance  coil,  3  amperes  being  carried  by  the  one,  5  by  the  other. 

lamp  totals  9  ohms.  The  pressure  necessary  to 
force  the  3  amperes  through  this  is,  of  course,  3X9 
or  27  volts. 


This  municipal  Christmas  tree  in  a  California  city  was  lighted  by  70 
lamps  in  parallel.  Each  lamp  took  .23  ampere  and  the  circuit  voltage 
was  110  at  the  tree.  The  current  was  brought  from  a  transformer  on 
two  wires  of  .12  ohm  each.  Can  you  calculate  the  resistance  of  each 
lamp,  the  combined  resistance  of  all  of  them,  the  voltage  drop  in  the 
wires   and  the  voltage   at  the   transformer? 

The  voltage  drop  in  any  conductor  equals  the 
pressure  required  to  force  the  current  through  its 
resistance. 


12 


APPLIED     ELECTRICITY 


The  drop  between  a  generator  and  its  load  de- 
pends, then,  upon  both  the  Hiie  resistance  and  the 
number  of  amperes. 

Resistances  in  Multiple. — When  several  lamps 
are  located  in  one  lighting  fixture  they  are  generally 
not  connected  in  series  with  each  other.     Though 


Two  400-horsepower  motors  are  connected  in  multiple  on  the  same  power 
circuit.  They  are  furthermore  "direct  connected"  to  the  same  shaft,  so 
that  the  load  is  divided  equally  between  them  and  the  motors  take  equal 
currents.  They  lift  the  mine  hoist  at  the  South  Eureka  gold  mine  on  the 
Mother  Lode  in  California. 

they  are  all  governed  by  one  wall  switch,  any  lamp 
can  be  burned  out  or  unscrewed  without  affecting 
the  others.  In  a  series  circuit  this  could  not  be 
done. 

In  Fig.  4  we  have  a  simple  example  of  two  re- 
sistances, a  lamp  and  a  coil,  connected  in  "multiple" 
with  each  other;  that  is,  connected  so  that  the  cur- 
rent flowing  through  the  ammeter  divides  and  part 
goes  through  the  lamp  and  part  through  the  coil. 


SERIES    AND    MULTIPLE     CIRCUITS 


13 


Either  can  be  disconnected  without  stopping  the 
current  in  the  other.  This  arrangement  is  some- 
times spoken  of  as  a  "shunt"  connection  or  a  "paral- 
lel" connection. 

Suppose  that  there  is  a  pressure  of  30  volts 
between  the  line  wires  at'  point  E,  close  to  the  load. 
This  is  a  measure  of    the  effort    to  send    current 


These  two  fans  operate  in  multiple  on  the  same  circuit.  Hence  either  can 
be  turned  off  without  stopping  the  other.  If  the  line  current  is  5  amperes 
at  110  volts  when  both  are  operating,  what  is  the  voltage  and  amperage 
for  each  motor?  What  is  the  resistance  of  a  heater  which  takes  as  much 
current  as  three   fan   motors  ? 


through  the  lamp  and,  if  the  lamp  has  10  ohms  re- 
sistance, it  will  carry  a  current  of  3  amperes.  The 
same  pressure  tends  to  send  current  through  the 
coil,  and,  if  that  has  6  ohms,  it  will  carry  5  amperes. 
The  ammeter  will  read  the  sum  of  these  two  cur- 
rents, 8  amperes. 


14  APPLIED     ELECTRICITY 

In  a  multiple  circuit  the  voltage  is  the  same 
across  every  branch;  total  current  equals  the  sum 
of  the  branch  currents. 

Combined  Resistance. — As  the  combination  of 
resistances  shown  in  Fig.  4  takes  more  current  than 
either  one  alone,  we  realize  that  the  combination 
offers  less  resistance  to  current  flow  than  either  of 
its  parts.  Considering  the  lamp  and  coil  as  united 
into  a  piece  of  apparatus  with  terminals  at  B  andC, 
we  may  compute  its  resistance  from  Ohm's  Law  as 
volts  -^-  amperes    :    30  -^-  8  =  3.75  ohms. 

A  standard  way  to  calculate  is  to  assume  one 
volt  applied  to  the  circuit,  add  the  currents  which 
would  flow,  and  divide  their  sum  into  the  one  volt. 
Trying  this  on  Fig.  4  we  have  1/10  +  1/6  =  16/60, 
the  combined  current.  The  resistance  =  1-=- 16/60 
=  60/16  =  3.75  ohms. 

In  a  multiple  circuit  combined  resistance  is  less 
than  ohms  in  any  one  branch;  combined  resist- 
ance =^1-^  sum  of  reciprocals  of  branch  resistances. 


Ill 


POWER— LOSSES— EFFICIENCY 

In  stating  the  "power"  of  a  motor  we  tell  not 
only  how  much  the  machine  can  do  but  also  how 
quickly  it  can  do  it.  A  single  horse  is  able  to  haul 
an  automobile  to  the  top  of  a  certain  long  hill,  but 
it  takes  a  40  horsepower  engine  to  perform  the  work 
in  five  minutes.  We  have,  then,  two  factors  which 
determine  the  amount  of  power,  the  force  required 
and  the  rate  at  which  it  drives  the  load. 


Fig.  5 — Metering  electric  power  with  a  watt  meter.  The  reading  depends 
both  upon  the  volts  (see  connection  to  the  pressure  coil,  P)  and  upon 
the  amperes    (see  connection  to  the  current  coil,   C). 


In  an  electric  circuit  we  have  what  corresponds 
to  a  force  (the  "voltage''  or  "pressure")  and  a  quan- 
tity of  electricity  which  is  driven  through  the  wires 
by  this  electromotive  force.  The  rate  at  which  the 
electricity  is  carried  is  indicated  by  the  number  of 
amperes  of  current,  or  "coulombs  per  second."  Both 

15 


16  APPLIED     ELECTRICITY 

the  voltage  and  the  current,  then,  are  factors  in  the 
power  necessary  to  force  electricity  around  a  circuit. 

It  has  been  agreed  that  the  simplest  practical 
unit  of  power  would  be  that  required  to  drive  one 
ampere  by  a  pressure  of  one  volt.  This  unit  is 
called  the  "watt"  in  honor  of  the  man  who  gave  the 
world  its  first  notions  of  power.  Then  if  one  volt 
drives  current  at  the  rate  of  3  amperes,  the  power  is 
3  watts.  If  it  takes  2  volts  to  drive  3  amperes,  the 
power  is  twice  as  much,  or  6  watts.  In  general,  the 
number  of  watts  equals  the  product  of  the  number 
of  volts  times  the  number  of  amperes. 

This  statement  holds  good  in  direct  current  cir- 
cuits and  in  those  alternating  current  circuits  which 
include  no  coils,  condensers  or  long  parallel  lines. 
With  one  or  more  of  these,  there  are  reactions  which 
reduce  the  power  requirement  below  the  number  of 
"volt-amperes."  The  following  expression  is  correct 
in  all  cases: 

Watts  --=  volts  X  amperes  X  power  factor, 
where  the  "power  factor"  is  equal  to  one  or  less, 
generally  expressed  as  a  percentage.  It  varies  from 
60%  to  90%  in  most  cases  of  alternating  current 
motors  and  transformers,  and  is  100%  for  heaters, 
resistance  boxes,  incandescent  lamps,  etc.,  and  for 
all  direct  current  circuits. 

Watts,  Kilowatts  and  Horse  Power. — ^The  direct 
current  load  carried  by  an  average  power  house  may 
be  2000  amperes  at  550  volts.  The  output  of  the  ma- 
chines, then,  equals  550X2000  =  1,100,000  watts. 
This  is  too  large  a  number  to  be  handled  conven- 
iently, and  it  is  customary  in  all  such  cases  to  use 
a  larger  unit,  namely,  the  "kilowatt,"  which  equals 
1000  watts.  Then  the  load  above  is  expressed  as 
1100  kw.   (kilowatts).    The  general  formula  is: 

Kilowatts  =  volts  X  amperes  X  power 
factor  -^  1000. 

It  is  perfectly  feasible  to  measure  the  power  in 
electric  circuits  in  terms  of  the  "horse  power," 
which  was  invented  primarily  for  such  machines  as 


POWER— LOSSES— EFFICIENCY  17 

the  steam  engine.  It  has  been  established  by  calcu- 
lation and  measurement  that  one  horsepower  equals 
746  watts,  or  approximately  %  kw.  Thus  it  is  pos- 
sible to  transfer  quantities  from  one  system  to  the 
other  with  little  difficulty: 

20  hp.  (horsepower)  =  %  of  20,  or  15  kw.,  and 

50  kw.  =  4/3  of  50,  or  66.7  hp.  (approximately) . 


Among  the  many  household  uses  for  electric  energy,  one  of  the  most 
convenient  is  illustrated  here.  With  a  certain  piece  of  sewing  in  the 
machine  the  motor  was  found  to  take  a  current  of  4  amperes,  while  a 
watt  meter  (connected  as  in  Fig.  5)  read  .12  kw.  What  was  the  voltage 
of  the  d.c.   circuit  which   supplied  the  current? 

Power  Measurement. — The  horsepower  output 
of  a  gas  engine  is  conveniently  measured  mechan- 
ically by  making  it  move  a  load.  One  horsepower  is 
required  to  lift  one  pound  at  the  rate  of  550  feet  per 
second,  or  to  lift  550  lbs.  one  foot  per  second.  For 
calculation, 

Hp.  =  pounds  pull  X  f^^t  per  second  -f-  550. 

Electric  power  may  be  measured  either  by 
means  of  a  wattmeter,  connected  as  shown  in  Fig.  5, 
or  by  a  combination  of  ammeter,  voltmeter  and 
power  factor  meter.  The  power  factor  meter  is,  of 
course,  not  used  with  circuits  which  are  known  to 
have  100%  power  factor.  The  wattmeter  has  both 
a  "pressure  coil"  (terminals  at  P  in  Fig.  5)  and  a 
"current  coil"  (terminals  at  C),  and  their  reaction 
upon  each  other  determines  the  reading,  which  is 
thus  proportional  to  both  volts  and  amperes.     On 


i5i_2 


POWER— LOSSES— EFFICIENCY  19 

a.c.  (alternating  current)  circuits  the  wattmeter  is 
able  to  take  account  of  power  factor  also. 

Losses  and  Efficiency.— The  voltage  at  the  gen- 
erator end  of  a  short  d.c.  (direct  current)  trans- 
mission line  is  250  volts.  At  the  receiving  end  a 
voltmeter  reads  230  and  an  ammeter  reads  50.    The 


This  heater  has  a  resistance  of  27.5  ohms.  What  current  does  it  take  on 
a  110  volt  circuit?  How  many  watts  of  power?  How  many  kw.  ?  How 
many  electrical   horse   power? 

voltage  drop  is  then  20  volts.  The  power  given  the 
line  by  the  generator  =  250  X  50  =  12500  watts  or 
12.5  kw.  That  delivered  by  the  line  =  230  X  50  = 
11500  watts  or  11.5  kw.  There  is  a  loss  of  power 
of  one  kw.  which  is  used  up  in  forcing  the  current 
through  the  wires. 

Such  a  loss  always  occurs,  and  it  results  in  heat- 
ing the  wires,  just  as  the  energy  expended  in  mov- 
ing a  train  changes  to  heat  in  the  parts  where  there 
is  friction. 

It  is  possible  to  calculate  the  watt  loss  in  a  d.c. 
circuit  by  multiplying  the  voltage  drop  in  the  line 
by  the  current,  as  the  watts  in  any  part  equal  volts 
across  that  part  times  amperes.  Thus  in  the  exam- 
ple above  we  find  20  X  50  =  1000  watts.    Further- 


20  APPLIED     ELECTRICITY 

more,  since  the  volt  drop  equals  ohms  X  amperes, 
the  watt  loss  =  ohms  X  amperes  X  amperes,  or  the 
product  of  resistance  times  the  square  of  the  cur- 
rent. This  last  is  a  most  important  relation,  and  it 
applies  to  all  circuits,  both  d.c.  and  a.c. 

When  we  speak  of  the  "efficiency"  of  a  trans- 
mission system  we  have  in  mind  a  comparison  of  the 
power  delivered  with  power  put  into  the  line  at  the 
generating  end.  Strictly,  the  efficiency  is  the  num- 
ber found  by  dividing  the  "output''  of  the  line  by  the 
"input."  As  an  example,  consider  a  system  which 
delivers  9,000  kw.,  the  current"  being  800  amperes 
and  the  total  line  resistance  3  ohms.  The  loss  = 
3  X  800  X  800  =  1,920,000  watts,  or  1,920  kw.  The 
input  to  the  line  =  9,000  +  1,920  =  10,920  kw.  Then 
the  efficiency  of  transmission  =  9,000  -f-  10,920  = 
.824,  which  is  82.4%. 

A  mechanical  machine,  such  as  a  waterwheel, 
may  give  out  40  h.p.  while  receiving  50  h.p.  from 
the  water.  Its  efficiency  =  40  -f-  50  ==  80  % .  No 
machine  has  ever  been  made  with  efficiency  as  great 
as  100%,  for  some  of  the  energy  put  in  is  always 
lost  by  friction.  No  transmission  of  electricity  is 
100%  efficient,  for  energy  is  always  converted  into 
heat  in  overcoming  resistance. 


ELECTROMAGNETS— TRANSFORMATION 
OF  ENERGY 

A  soft  iron  bar  with  a  coil  of  wire  around  it 
becomes  a  magnet  when  current  flows  through  the 
wire.  The  magnetism  disappears  when  the  current 
stops,  so  that  whatever  had  been  picked  up  now 
falls  away.  A  similar  "electromagnet"  may  be  made 
without  any  iron,  but  the  pull  it  exerts  is  far  less. 

Magnetic  effects  are  explained  on  the  theory 
that  an  electromagnet  or  a  permanent  steel  magnet 
produces  "lines  of  force''  which  issue  from  one  end 
or  "pole"  and  return  to  the  other  end,  and  then 
pass  through  the  instrument  itself.  Thus  eacK  line 
of  force  is  a  complete,  closed  curve. 

The  end  of  the  magnet  out  of  which  the  lines 
come  is  called  the  "north  pole"  for  it  is  found  that, 
whenever  it  is  so  supported  as  to  be  free  to  turn 
(as  by  floating  it  upon  a  cork  or  balancing  it  upon 
a  pivot),  this  end  turns  toward  the  north.  The 
force  lines  (often  called  "magnetic  flux")  enter  the 
magnet  at  the  "south  pole,"  after  passing  through 
the  air  or  any  iron  or  steel  objects  in  the  neigh- 
borhood. 

Magnetic  flux  runs  through  all  substances,  but 
far  more  easily  through  iron  and  steel  than  any 
other  material.  Thus  a  current  flowing  in  a  simple 
coil  produces  lines  of  force,  but  with  an  iron  core  in 
the  coil  many  more  lines  are  found.  And  if  an  iron 
path  is  provided  for  the  lines  outside  the  magnet,  so 
they  do  not  have  to  go  through  any  other  material, 
a  large  flux  can  be  produced  with  a  small  current. 

21 


22  APPLIED     ELECTRICITY 

Hence,  electromagnets  are  often  made  in  the  shape 
of  a  horseshoe,  and  iron  paths  are  provided  for  the 
external  flux  in  such  electromagnetic  machines  as 
generators  and  motors. 

The  amount  of  flux  produced  by  a  magnet  de- 
pends upon  the  number  of  turns  in  the  coil  and  upon 
the  current,  as  well  as  upon  the  material  the  lines 
traverse.  It  is  found  that  8  amperes  through  20 
turns  give    exactly    the    same  flux  as  40  amperes 


This  is  a  hand  magnet 
used  in  various  ways 
such  as  the  handling  of 
hot  castings,  removing 
iron  and  steel  from 
materials  for  making 
solder  and  recovering 
nails    from    sweepings. 


through  4  turns.  In  other  words,  the  product  of 
amperes  X  turns  (which  is  called  the  "ampere- 
turns'")  determines  the  tendency  to  produce  flux, 
while  the  number  of  lines  actually  set  up  depends 
also  upon  the  nature  of  the  magnetic  path. 

Ammeters. — Applications  of  electromagnetism 
are  found  in  the  ammeters  used  for  measuring  cur- 
rents on  switchboards  and  elsewhere.  One  type  is 
illustrated  in  Fig.  6,  which  shows  the  d'Arsonval 
movement  found  in  one  type  of  ammeter.  When  the 
current  to  be  measured  flows  around  the  coil  at- 
tached to  the  pointer,  the  coil  becomes  a  magnet 
with  each  face  one  pole.  These  are  attracted  to  the 
opposite  sides  of  the  permanent  steel  magnet  con- 
stituting the  frame,  which  causes  the  coil  to  turn 
against  the  restraint  of  springs.  If  the  current  in- 
creases, the  turning  effort  becomes  stronger,  so  that 
the  pointer  is  carried  farther  along  the  scale. 


ELECTROMAGNETS 


23 


The  KOowatt  Hour. — If  a  man  uses  3  kw.  for 
four  hours  he  makes  only  half  as  much  demand  on 
the  power  company  as  if  he  used  3  kw.  for  eight 
hours,  and  he  gets  only  half  as  much  work  done  by 
his  motor.  In  one  case  he  uses  4  X  3  or  12  "kilowatt- 


Fig.   6. — Current   entering  the  ammeter  through    the   spiral    spring    makes 
the  coil  and  its  iron  core  an  electromagnet 


hours"  of  electrical  energy,  and  in  the  other  case 
8X3  or  24  kw-hr.  The  cost  of  electric  service 
depends  on  both  the  power  and  the  time;  the  usual 
custom  is  to  base  the  charge  on  the  "kilowatt-hour," 
which  is  the  energy  supplied  in  one  hour  by  one  kw. 
The  retail  price  of  one  kw-hr.  varies  from  one  to  5 
cents  for  heating,  cooking  and  motors,  and  from  6  to 
15  cents  for  lighting.  Energy  is  sometimes  sold  by 
the  horsepower-hour  (%  of  a  kw-hr.),  and  some- 
times by  the  horsepower-year  (especially  for  pump- 
ing irrigation  water). 


24  APPLIED     ELECTRICITY 

At  6  cents  per  kw-hr.  $1.80  will  buy  30  kw-hr. 
The  power  may  be  30  kw.  for  1  hour;  or  10  kw.  for 
3  hours;  or  6  kw.  for  5  hours;  or  l/^  kw.  for  60 
hours;  or  4  kw.  for  3  hours  and  9  kw.  for  2  hours. 
Evidently  a  statement  of  a  number  of  kw-hr.  does 
not  tell  anything  at  all  about  the  number  of  kw.  (or 
power).  Instruments  that  measure  kw-hr.  are  not 
"watt-meters"  but  "watt-hour  meters." 


WESTERN  ELECTRIC  HOT  PLATE  AND  KLAXON  HORN 

Here  are  two  devices  for  transforming  electric  energy  into  other  forms — 
sound  in  the  Klaxon  horn  and  heat  in  the  hot  plate.  The  latter  takes 
4  amperes  on  a  220  volt  circuit — how  many  British  thermal  units  will  it 
give  out  in  an  hour?  If  one-half  of  the  heat  escapes  to  the  air,  how  hot 
will  a  gallon  of  water  (8  lbs.)  get  in  half  an  hour  if  its  temperature  is 
60°    when   it   is   set  upon  the  hot  plate? 


If  you  know  the  kw-hr.  and  the  number  of 
hours,  you  can  find  the  average  kw.  In  the  example 
above,  if  it  is  given  that  the  30  kw-hr.  are  used  in 
5  hours  we  can  say  that  the  average  power  is  6  kw. 
— but  it  may  be  4  kw.  for  3  hours  and  9  for  2  hours. 
Note  the  particular  meaning  of  the  word  "average** 
here;  average  kw.  =  number  of  kw-hr.  divided  by 
the  number  of  hours. 

Heat  Energy. — It  has  been  found  by  numerous 
careful  experiments  that  when  one  kw-hr.  of  elec- 
trical energy  is  used  up  in  overcoming  "electrical 
friction**  or  resistance,  a  certain  definite  amount  of 
heat  is  developed,  namely,  enough  to  heat  3,412 
pounds  of  water  one  degree  hotter  (by  Fahrenheit 
thermometer).  This  is  generally  expressed  by  say- 
ing that  1  kw-hr.  =  3,412  "British  Thermal  Units** 
or  3,412  B.t.u.  Also  when  one  kw-hr.  of  mechanical 
energy  (1 1/3  h.p.  hours)  is  used  up  in  overcoming 


g  S      -5  ft  S  ^  ®.  .  2>  S  XJ  4>  oj  c  ^ 

■Ml~  §  *"-"3  »  «  5-£  £  -^  «.£  0,=^  St! 


26 


APPLIED     ELECTRICITY 


friction  3,412  B.t.u.  of  heat  is  developed.    One  horse- 
power-hour similarly  gives  2,545  B.t.u. 

In  a  steam  or  gas  engine  it  is  possible  to  meas- 
ure the  heat  developed  by  the  fuel  and  the  heat 
wasted  in  the  exhaust,  radiation,  etc.  The  loss  of 
heat  is  always  less  than  the  heat  developed,  the  dif- 
ference being  a  certain  definite  number  of  B.t.u.  in 
each  case.    It  is  found  that  this  difference,  divided 


INSIDE   THE   FARM  HOUSE 

This  installation  of  an  electric  range  and  an  electric  water  heater  in  a 
ranch  home  in  California  uses  $6.00  worth  of  electric  energy  per  month 
at  a  rate  of  3  cents  per  kw-hr.  What  was  the  average  current  taken,  if 
the  apparatus  was  used  on  a  110  volt  circuit  5  hours  a  day  for  30  days? 

by  2,545,  gives  the  number  of  h.p.  hours  of  mechan- 
ical energy  developed.  Or,  the  ''useful"  B.t.u.  divided 
by  3,412  =  the  number  of  mechanical  kw-hr.  Thus 
it  is  proved  that  3,412  B.t.u.  can  be  changed  to 
1  kw-hr.,  or  1  kw-hr.  can  be  changed  to  3,412  B.t.u. 
Mechanical  energy,  electrical  energy,  and  heat  are 
simply  three  forms  of  the  same  thing;  by  various 
devices  we  can  make  energy  take  any  form  desired. 

Efficiency  of  Transformation. — In  many  trans- 
formations of  energy,  there  is  a  "loss"  of  some  of 


ELECTROMAGNETS  27 

the  energy — loss  in  the  sense  that  the  energy- 
changes  into  some  form  that  is  not  desired,  or  goes 
to  some  place  where  it  is  not  wanted.  For  example, 
in  a  motor  we  wish  electrical  energy  to  be  converted 
to  mechanical,  but  inevitably  some  energy  goes  into 
heat  through  friction,  resistance  in  the  wires,  etc. 
If  the  motor  has  an  efficiency  of  80%,  20%  of  the 
electrical  input  is  expended  in  undesired  ways.  Sim- 
ilarly, in  an  electric  water  heater  we  desire  the  con- 
version of  the  electrical  energy  into  heat  in  the 
water,  but  some  heat  is  sure  to  escape  to  the  sur- 
rounding air,  material  of  the  container,  etc.  The 
efficiency  of  such  a  device  =  useful  B.t.u.  -^  total 
B.t.u.  produced  from  the  electrical  energy ;  or  eff .  = 
useful  B.t.u.  -^  3,412  X  kw-hr.  expended.  On  the 
other  hand,  an  electric  air  heater  in  a  room  has  per- 
fect or  100%  efficiency,  for  all  the  heat  must  get  into 
the  air  and  objects  where  it  is  wanted. 

Engines  which  are  used  for  the  purpose  of  con- 
verting heat  into  mechanical  energy  are  compara- 
tively inefficient.  A  large  part  of  the  heat  given  to 
them  is  sent  out  again  as  heat  and  not  as  work. 
Most  of  it  goes  out  through  the  exhaust,  but  in 
many  cases  (as  in  gas  engines)  a  large  part  of  the 
heat  is  passed  out  through  the  cylinder  walls  to  the 
cooling  water.  The  efficiency  of  steam  and  gas  en- 
gines in  practice  varies  from  10%  to  30%. 


WIRE   CALCULATIONS 

Wires  used  to  carry  electricity  are  usually  of 
copper,  aluminum  or  iron.  In  the  United  States 
copper  and  aluminum  wires  are  made  in  various  sizes 
according  to  an  arbitrary  set  of  dimensions  known  as 
the  "Brown  &  Sharpe  (or  American  Standard)  Wire 


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Fig.  7.  —  Connecting 
block  for  an  interior 
telephone  system.  If 
each  wire  is  No.  18, 
what  is  the  resistance 
of  200  feet  of  the  cable 
if  all  the  wires  are  con- 
nected together  at  each 
end? 


Gage."  A  wire  known  as  No.  5,  for  example,  has  a 
diameter  of  .1819  inch,  and  No.  10  has  a  diameter 
of  .1019  inch.  The  largest  size  in  this  gage  is 
No.  0000,  the  next  is  No.  000,  then  00,  then  0,  then 
Numbers  1  to  40,  of  which  the  last  is  the  smallest 
wire  (.00315  inch  diameter). 


28 


WIRE    CALCULATIONS  29 

It  is  customary  to  express  the  diameter  in 
"mils"  (one  mil  =  .001  inch)  rather  than  in  inches. 
Then  No.  5  wire  is  181.9  mils  thick,  and  No.  10  is 
101.9  mils  thick.  The  cross  section  area  of  a  wire 
can  not  be  expressed  conveniently  in  square  inches; 
it  is  found  preferable  to  use,  instead,  the  "circular 
mil"  (cm.)  as  a  unit.  For  wires  and  cables  larger 
than  No.  0000  the  size  is  designated  by  their  circular 
mil  area.  The  diameter  of  a  solid  wire  is  found  by 
taking  the  square  root  of  its  cm.  area.  When  the 
diameter  is  known,  the  cm.  area  is  found  by  squar- 
ing the  number  of  mils  in  the  diameter. 

Wire  tables  are  found  in  all  electricians'  text 
books  and  hand  books.  From  these  one  can  quickly 
determine  the  characteristics  of  copper  wire  of  any 
size.  The  tables  differ  somewhat  in  arrangement, 
but  practically  all  of  them  give  the  diameter  and  the 
section  area  for  each  numbered  size,  together  with 
the  resistance  and  the  weight  of  1000  feet  of  wire. 
The  partial  table  on  p.  32  gives  figures  for  a  number 
of  sizes  of  copper  and  aluminum  wire. 

If  one  knows  that  the  resistance  of  one  foot  of 
36  wire  is  .414  ohm,  it  is  easy  to  compute  the  resist- 
ance of  any  length.  Six  feet,  for  instance,  will  have 
6  X  .414  or  2.484  ohms,  for  we  have  six  resistances 
of  .414  ohm  each  connected  in  series.  Then  the 
resistance  of  220  feet  of  No.  18  copper  wire  is  220 
times  1/1000  of  6.374  (see  table),  for  6.374  ohms  is 
the  resistance  of  1000  feet. 

The  weight  of  any  length  of  bare  wire  is  figured 
in  exactly  the  same  way  from  the  tabulated  figures 
of  the  lbs.  per  thousand  feet.  Manufacturers  and 
dealers  supply  with  price  lists  tables  of  the  weights 
of  wires  insulated  in  various  ways. 

In  the  table  below  the  resistances  are  given  for  a 
temperature  of  68°  Fahrenheit.  For  any  other  tem- 
perature the  figures  are  incorrect,  since  the  resist- 
ance of  metals  increases  with  rising  temperature. 
For  copper  and  aluminum  the  ohms  increase  about 
0.2%  for  each  degree,  so  that  at  88°  the  resistance 
is  4%  greater  than  indicated  by  the  table. 


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WIRE    CALCULATIONS 


31 


Allowable  Current.  —  Wires  carrying  current 
become  heated,  and  the  temperature  rises  until  the 
loss  of  heat  to  the  surroundings  equals  the  heat 
developed.  Then  the  temperature  remains  station- 
ary.   The  hotter  the  wire  becomes,  the  more  heat  it 


For   replacing    carbon    lamps    in    "lamp   banks"  used    as    rheostats,    these 

resistor  units   are  being  used.     This   particular  one   absorbs    60    watts   at 

120  volts.  What  length  of  No.  36  copper  wire  has  an  equal  resistance? 
What  length  of  No.  36  nichrome? 

can  give  off  per  second,  so  that  (unless  it  is  sur- 
rounded by  a  non-conductor  of  heat  like  asbestos) 
the  temperature  of  a  wire  depends  upon  the  watt 
loss  in  it,  which  in  turn  depends  upon  the  current. 

The  insulation  upon  conductors  used  in  inside 
wiring  will  deteriorate  if  exposed  to  heat,  and  hence 
the  temperature  rise  of  the  wires  must  be  strictly 
limited.  The  National  Board  of  Fire  Underwriters 
issue  in  the  '^National  Electric  Code*'  a  table  of  the 
current  to  be  allowed    in    copper    wires    used  for 


This  vacuum  cleaner 
hose  must  carry  a  large 
current  of  air  with  lit- 
tle resistance.  Hence  it 
is  made  as  large  as 
practicable.  What  would 
be  the  loss  of  pressure 
in  a  similar  tube  twice 
as  long? 


interior  wiring.  This  is  given  in  the  wire  table 
accompanying  this  lesson.  Note  that  the  current 
in  rubber  insulated  wire  must  be  kept  smaller  than  in 
wires  with  other  insulations,  on  account  of  the  fact 
that  rubber  deteriorates  at  lower  temperature  than 
other  insulations. 


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WIRE    CALCULATIONS  33 

Aluminum,  Iron,  and  Other  Wires. — Since  alum- 
inum has  a  lower  conductivity  than  copper,  wires 
of  aluminum  must  be  larger  than  copper  wires  for 
the  same  current.  But  its  density  is  so  much  lower 
than  that  of  copper  that  the  larger  aluminum  wire 
weighs  much  less  than  the  copper  it  replaces. 
Knowing  the  size  of  copper  needed  for  a  certain 
load,  one  can  select  an  aluminum  wire  two  sizes 
larger  (in  the  B.  &  S.  gage)  and  figure  the  weight  as 
1/^  that  of  the  copper. 

Iron  wire,  used  to  some  extent  for  telephone 
lines,  has  much  higher  resistance  than  copper  or 
aluminum.  It  is  manufactured  in  sizes  different 
from  those  in  the  B.  &  S.  gage,  the  system  followed 
being  called  the  "Birmingham  wire  gage."  The  grade 
known  as  "Best  Best"  (B.B.)  has  the  following 
characteristics  in  two  sizes  much  used : 

No.   (B.W.G.)  CM.  Lbs.  per  mile  Ohms  per  mile 

9    21904  314  17.84 

14    6889  99  56,56 

Suppose  that  No.  14  B.B.  wire  is  to  be  used  to 
make  a  resistance  for  an  arc  lantern.  If  the  line 
voltage  is  110,  the  pressure  across  the  arc  70  volts, 
and  the  current  to  be  used  is  18  amperes,  how  many 
pounds  of  wire  should  be  bought?  The  drop  in  the 
wire  =  110 — 70 --40  volts;  the  resistance  =  volts/ 
amperes  =  40/18  =  2.22  ohms.  The  table  gives  the 
ohms  per  mile  for  No.  14  as  56.56,  hence  the  ohms 
per  foot  =  56.56/5280  =  .0107 ;  2.22/.0107  =207,  the 
number  of  feet  of  wire.  The  lbs.  per  mile  is  given  as 
99;  99/5280  =  .0187  lbs.  per  foot;  hence  our  wire 
weighs  207  X  .0187  lbs.  or  3.88  pounds. 

When  resistance  is  desired,  it  is  usual  to  take 
instead  of  iron  a  wire  of  some  composition,  such  as 
"German  Silver,^'  "Climax,"  or  "Nichrome.*'  The 
German  silver  is  an  alloy  of  copper,  zinc  and  nickel ; 
most  other  special  resistance  materials  are  alloys  of 
steel  with  nickel,  etc. 

These  materials  are  made  into  flat  and  square 
conductors  and  round  wires,  the  B.  &  S.  gage  being 


34  APPLIED     ELECTRICITY 

genierally  adopted  for  the  round  wires.  To  calculate 
the  resistance  of  any  wire,  find  the  ohms  of  a  piece 
of  copper  wire  of  the  same  size  and  length,  and  mul- 
tiply by  the  proper  multiplier :  for  Nichrome  multi- 
ply by  60;  for  Climax  multiply  by  50;  for  German 
Silver  (ordinary,  with  18%  nickel)  multiply  by  20; 
for  Brass  multiply  by  4.4;  for  Aluminum  Bronze 
multiply  by  7.5;  for  Steel  multiply  by  8.3.  To  find 
the  weight  of  Nichrome  wire  multiply  the  weight 
of  a  similar  copper  wire  by  .92 ;  for  Climax  multiply 
.by  .92;  for  18%  German  Silver  multiply  by  .95;  for 
Steel  multiply  by  .88. 


VI 


THE  GENERATOR 


Faraday's  Discovery. — As  shown  by  the  electro- 
magnet, an  electric  current  can  produce  magnetism ; 
it  was  to  be  expected  that  electric  current  could  be 
produced  from  a  magnet.  After  many  fruitless 
efforts  Faraday  in  1831  discovered  how  to  do  it ;  with 
the  arrangement  shown  in  Fig.  8  he  succeeded  in 
causing  a  flow  of  current  through  the  sensitive  am- 
meter whenever  he  moved  the  magnet  into  or  out 
of  the  coil.  The  current  stopped  as  soon  as  the 
movement  stopped;  leaving  the  magnet  in  the  coil 
produced  no  current.  But  moving  the  coil  off  the 
magnet  while  the  latter  was  held  still  caused  the 
current  to  flow  just  as  if  the  magnet  itself  had 
moved. 

The  effects  produced  by  moving  the  magnet  or 
the  coil  may  be  said  to  be  caused  by  the  cutting  of 
the  wires  by  lines  of  force.  As  soon  as  the  cutting 
stops,  the  circuit  is  dead.  The  current  through  the 
ammeter  depends  upon  the  resistance  of  the  connect- 
ing wires  as  well  as  upon  the  motion  of  the  magnet ; 
so  it  is  better  to  replace  the  ammeter  by  a  voltmeter 
and  make  further  investigations  of  the  phenomenon 
on  the  basis  of  electromotive  force. 

This  e.m.f.  is  said  to  be  "induced''  by  the  mag- 
netic lines,  just  as  pieces  of  iron  acquire  ''induced 
magnetism''  in  the  neighborhood  of  a  magnet. 

35 


36 


APPLIED     ELECTRICITY 


Experiments  with  coils  of  various  numbers  of 
turns,  with  magnets  moving  slowly  and  rapidly,  and 
with  magnets  of  different  strengths,  lead  to  this 
conclusion:  The  induced  e.m.f.  depends  upon  the 
number  of  lines  of  force,  the  speed  with  which  they 
cut  the  wire,  and  the  number  of  turns  of  wire  in 
series  .  (A  strong  magnet  has,  of  course,  more  lines 
of  force  than  a  weak  one.)    It  was  found  possible  to 


'   '  / 

s' 

\  \  \ 

V'>. 

N 

1 1  >' 

Fig.  8. — ^The  details  of  Faraday's  arrangement  for  causing  a  flow  of  cur- 
rent through  an  ammeter  by  moving  the  magnet  into  and  out  of  the  coil. 

get  a  definite  numerical  statement,  as  will  be  ex- 
plained later  in  connection  with  the  voltage  of 
generators. 

Faraday's  apparatus  always  produced  an  alter- 
nating voltage;  pulling  out  the  magnet  induced  an 
e.m.f.  opposite  to  that  caused  by  putting  it  in. 
Inserting  the  S  pole  induced  a  voltage  opposite  to 
that  caused  by  inserting  the  N  pole.  The  only  way 
to  get  a  current  through  the  ammeter  of  Fig.  8 
continuously  in  the  same  direction  is  to  reverse  the 
connections  with  every  movement  of  the  magnet. 

The  A.C.  Generator. — On  account  of  the  mechan- 
ical advantages  of  rotary  over  reciprocating  motion, 
generators  are  built  commercially  with  either  the 
magnets  or  the  coils  revolving.  Fig.  9  gives  a  gen- 
eral idea  of  the  arrangement  of  a  "revolving  field'' 
alternator.  We  have  here  a  magnet  turning  on  a 
shaft  so  that  the  lines  of  force  close  to  its  poles  cut 
across  the  stationary  wires  A  and  B.  The  N  pole 
induces  in  wire  A  a  voltage  directed  toward  the  back 


THE    GENERATOR 


37 


of  the  machine,  while  in  B  the  S  pole  sets  up  an 
e.m.f.  in  the  forward  direction.  The  wires  are 
joined  at  the  rear  and  connected  to  a  lamp  in  front, 
which  at  the  instant  shown  has  current  flowing  in 
it  from  right  to  left.  When  the  poles  are  turned  far 
enough  to  exchange  places,  the  electromotive  force 
is  again  induced  but  in  the  opposite  direction,  so 


Fig.  9.- 


-The  plan  of  the  "revolving  field"  alternator.     At  the  instant  shown 
the   lamp  has   current  in   it   from   right  to   left 


that  the  current  flows  from  left  to  right  in  the  lamp, 
having  stopped  when  the  magnet  was  halfway  be- 
tween the  two  positions.  Thus  is  produced  an  alter- 
nating current,  which  might  be  "rectified"  so  as  to 
flow  always  in  the  same  direction  through  the  lamp 
by  reversing  the  lamp  connections  twice  in  each  rev- 
olution of  the  magnet. 

The  **revolving  field''  usually  contains,  instead 
of  a  single  permanent  magnet,  a  number  of  strong 
electromagnets.  An  iron  ring  is  placed  around  the 
outside  of  the  "inductors"  (wires  cut  by  the  lines 
of  force)  so  that  the  flux  may  get  from  pole  to  pole 
with  as  little  difficulty  as  possible.  Thus  there  is 
provided  a  large  number  of  lines  of  force,  and  by 
turning  the  shaft  rapidly  it  is  possible  to  induce  sev- 
eral volts  in  one  inductor.  Finally,  a  number  of 
wires  are  connected  in  series,  so  that  the  generator 
produces  any  voltage  desired. 


38 


APPLIED     ELECTRICITY 


The  D.C.  Generator. — A  revolving  field  machine 
is  not  v^ell  adapted  for  producing  direct  currents. 
The  d.c.  generator  in  Fig.  10  has  therefore  a  station- 
ary field  and  revolving  "armature."  (The  armature 
is  the  structure  which  includes  the  inductors  and 
the  iron  core  to  which  they  are  attached ;  the  core  is 
omitted  from  Fig.  10  for  the  sake  of  clearness.) 


Fig.  10. — A  d.c.  generator  with  a  stationary  field  and  revolving  armature 
for  producing  a  direct  current.  In  the  position  shown  the  wire  A  has  induced 
in  it  a  voltage  directed  forward,  which  drives  current  out  of  the  armature 
through  the  sliding  contact  between  a  "segment"  (B)  and  a  brush  (C). 
The  segment  B  is  attached  to  the  armature  and  revolves  with  it,  so  that 
when  A  reaches  a  position  close  to  the  N  pole  it  is  in  contact  with  the 
other  brush,  and  segment  E  has  come  around  to  brush  C.  Thus  the  wire 
passing  the  S  pole  always  sends  current  out  to  brush  C,  and  so  downward 
through  the  lamp,   L. 


With  this  simple  arrangement  the  voltage  drops 
to  zero  twice  in  every  revolution.  If  a  second  coil 
were  put  on  the  armature  at  right  angles  to  the  first 
coil,  it  would  have  a  strong  e.m.f.  when  the  first 
e.m.f.  is  zero.  With  the  ring  split  into  four  parts 
instead  of  two,  and  the  new  coil  connected  to  two  of 
the  segments,  a  fairly  uniform  pressure  could  be 
produced.  In  practice  many  coils  are  used  on  the 
armature  and  correspondingly  many  segments  are 
built  into  the  ''commutator"  as  the  split-ring  device 
is  called. 


THE    GENERATOR  39 

It  should  be  clear  that  a  d.c.  generator  pro- 
duces an  alternating  current  which  is  "rectified''  or 
made  into  direct  current  by  means  of  the  commu- 
tator. Many  d.c.  machines  are  made  with  more 
than  two  poles;  in  these  there  are  usually  as  many 
brushes  as  poles.  Carbon  is  the  material  used  in 
the  brushes,  chiefly  because  of  its  resistance  which 
is  useful  in  limiting  the  short  circuit  currents  which 


In  a  Columbia  River  salmon  cannery  this  6  kw.,  120  volt  d.  c.  generator 
produces  all  the  current  necessary  to  operate  the  lights  and  motors.  It  is 
driven  by  a  10  h.  p.  semi-Diesel  oil  engine.  What  is  the  efficiency  of  the 
generator  and  what  current  does  it  supply  when  fully  loaded?  How  many 
pounds  of  oil  are  used  in  8  hours  if  the  average  load  is  40  amperes,  the 
engine  efficiency  is  30%  and  the  fuel  gives  19000  B.  T.  U.   per  lb.? 

tend  to  flow  from  one  *'bar"  (segment)  of  the  com- 
mutator to  the  next  while  the  brush  is  touching 
both. 

Generator  Voltage. — It  is  customary  to  specify 
the  strength  of  the  magnets  used  in  generators  by 
the  number  of  lines  of  force  they  produce.  This 
system  is,  of  course,  based  on  an  arbitrary  standard, 
for  a  line  of  force  is  merely  a  convenient  term  to 
use  in  connection  with  magnets  and  it  has  no  tangi- 


THE    GENERATOR  41 

ble  existence.  In  practice  the  electromagnets  most 
used  have  from  100,000  to  several  million  lines 
issuing"  from  their  N  poles.  It  is  to  be  noted  that 
one  inductor  cuts  across  all  these  lines  once  when 
passing  the  N  pole  of  a  magnet  and  again  when 
passing  the  S  pole,  so  that  the  total  number  of  cut- 
tings during  one  revolution  equals  the  product  of  the 
number  of  poles  times  the  lines  per  pole. 

The  standards  of  pole  strength  and  electro- 
motive force  have  been  so  selected  that  one  volt  is 
the  e.m.f.  induced  by  cutting  one  wire  in  one  second 
by  100,000,000  lines  of  force.  Putting  inductors  in 
series  adds  their  voltages,  so  that 

Generator  Voltage  =  No.  Poles  Passed  per 
Second  X  Flux  per  Pole  X  No.  Wires  in 
Series  -f-  100,009,000. 

This  formula  gives  the  average  voltage  for  an 
alternating  or  fluctuating  current,  or  the  steady 
voltage  in  a  d.c.  machine.  The  number  of  wires  in 
series  may  be  the  total  number  of  inductors  on  the 
armature,  or  one-half  that  number,  or  one-fourth 
or  one-sixth,  or  less.  Direct  current  generators 
usually  have  enough  coils  and  commutator  segments 
to  make  the  voltage  practically  constant  at  the  value 
given  by  the  formula. 


VII 


ARMATURE  AND   FIELD  WINDINGS 

Of  all  the  types  of  generators,  the  simplest  is 
the  alternating  current  magneto,  which  is  used  for 
ringing  telephone  bells  and  for  ignition  in  gas 
engines.  The  magnetic  flux  is  supplied  by  one  or 
more  stationary  permanent  magnets  of  horseshoe 
shape.  The  armature  generally  consists  of  a  single 
coil  wound  upon  an  iron  core,  which  revolves  between 
the  magnet  poles.    One  end  of  the  winding  may  be 


Fig.   11. — Shunt  generator.     Coils  of  small  wire  on  the  magnet  poles  are 
connected   in   multiple   or   shunt   with   the   load. 

"grounded"  (connected  to  the  metal  of  the  arma- 
ture) while  the  other  is  connected  to  the  external 
circuit  through  a  sliding  contact. 

All  other  generators  have  electro-magnets  for 
producing  the  "magnetic  field"  or  flux.  The  wires 
which  carry  the  current  around  these  magnets  con- 
stitute the  "field  winding." 


42 


ARMATURE    AND     FIELD    WINDINGS  43 

The  magnets  are  ^'excited''  by  sending  direct 
current  through  these  windings,  the  current  being 
produced  either  by  the  generator  itself  or  by  some 
external  source.  Direct  current  generators  are  almost 
always  ''self  excited/'  while  a.c.  machines  are  "sepa- 
rately excited"  by  the  use  of  small  d.c.  generators 
called  "exciters."  A  few  alternators  have  special 
arrangements  for  producing  small  amounts  of 
direct  current,  thus  saving  the  expense  of  an  extra 
machine. 

In  Fig.  11  is  shown  the  simplest  arrangement  for 
self  excitation,  a  direct  current  machine  with  the 
field  winding  connected  in  piultiple  or  "shunt"  with 
the  load.  The  diagram  on  the  right  is  a  preferable 
way  to  represent  the  same  arrangement.  Many 
turns  of  fine  wire  are  used,  which  offer  enough  re- 
sistance to  limit  the  field  current  to  a  small  value, 
and  yet  give  sufficient  "ampere-turns"  almost  to 
saturate  the  iron  with  magnetism. 

When  the  machine  is  stopped  the  current  dies 
out  of  the  shunt  field  and  the  magnetism  disappears, 
with  the  exception  of  a  small  amount  which  is  known 
as  "residual  magnetism";  that  is,  the  iron  has  to  a 
slight  extent  the  characteristics  of  a  permanent 
magnet.  When  the  generator  is  again  brought  up 
to  its  running  speed,  it  is  found  that  a  low  voltage 
is  produced,  and  if  the  shunt  field  switch  is  then 
closed,  a  small  current  is  sent  through  the  coils. 
This  increases  the  magnetism  and  raises  the  voltage, 
which  comes  up,  little  by  little,  to  the  pressure  for 
which  the  machine  is  designed. 

To  control  the  electromotive  force  of  a  genera- 
tor, it  is  customary  to  insert  a  variable  resistance 
or  "rheostat"  in  series  with  the  field  winding.  Thus 
the  current  and  flux  can  be  altered  at  will,  and  hence 
the  voltage,  which  depends  on  the  strength  of  the 
magnets,  can  be  raised  or  lowered  within  wide  limits. 
(See  the  rheostat,  R,  in  Fig.  12.) 

An  additional  feature  of  the  field  winding  of 
most  d.c.  generators  is  shown  in  the  "series  winding" 
in  Fig.  12.  A  few  more  turns  of  wire  are  put  around 


r.  c 


35 


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00   S 

II 


t3    4) 


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O   ft 


1 

<  bo 


ARMATURE    AND    FIELD    WINDINGS 


45 


the  magnet  poles  and  this  wire  (which  is  made  of 
large  size)  is  connected  in  series  with  the  load.  The 
additional  magnetism  thus  produced  raises  the  gen- 
erated voltage  as  the  current    increases,  and  thus 


\QQStSir- 


Fig.  12. — Compound  wound  generator.  The  load  current  passing  through 
the  series  coils  tends  to  raise  the  generated  voltage  as  the  load  increases. 
The  rheostat  in  the  shunt  circuit  enables  the  operator  to  change  the 
voltage  at  will. 

compensates  for  the  increased  *'line  di^op'*  at  heavy 
loads.  A  generator  thus  equipped  is  said  to  be  "com- 
pound wound.'' 

The  field  coils  of  an  alternating  generator  are 
connected  to  the  exciter  through  a  rheostat.  If  the 
alternator  field  revolves,  it  is  necessary  to  get  the 


Voltage  regulation  by  the 
shunt  field  rheostat  is 
sometimes  not  sufficiently 
accurate.  This  series 
rheostat  and  ammeter  are 
used  to  keep  the  current 
constantly  equal  to  20 
amperes  in  the  600  watt 
gas  filled  lamp  used  in 
many  moving  picture 
projectors.  What  is  the 
lowest  possible  generator 
voltage   for   this   service? 


current  to  and  from  the  windings  through  sliding 
contacts  on  "slip  rings."  This  is  preferable  to  using 
sliding  contacts  for  the  generator  current,  which  is 
usually  at  high  voltage  and  of  much  greater  volume 
than  the  exciting  current. 

Armature  Winding. — Direct  current  generators 
usually  have  drum  shaped  armatures,  built  of  many 
thin  circular  leaves  or  "laminations"  of  iron  or  steel. 
The  wire  is  placed  in  slots  cut  lengthwise  along  the 


46 


APPLIED     ELECTRICITY 


cylindrical  surface  and  connected  at  numerous  points 
to  the  copper  commutator  bars. 

In  Fig.  13  is  shown  a  very  simple  winding  for  a 
'^bipolar"  (2  pole)  machine  having  6  slots  in  the 
armature,  12  inductors  and  6  commutator  bars.  The 
letter  B  indicates  the  bar  in  contact  with  one  of  the 
brushes — let  us  say  the  "positive''  brush,  or  the  one 
at  which  the  current  leaves  the  armature.  Then  the 
current  enters  the  winding  at  A,  and  flows  along  the 
wires  to  inductors  numbered  1  and  7,  both  of  which 


N 


Fig.  13. — Armature  of  bipolar  d.c.  generator  seen  from  end  of  shaft.  Note 
connections  of  inductors  (in  the  slots)  to  each  other  and  to  the  commutator 
bars.  Other  interconnections  at  the  other  end  of  the  armature  are  invis- 
ible in  this  view. 


are  under  the  influence  of  the  N  pole.  Obviously,  the 
machine  must  be  turning  in  such  a  direction  that 
voltage  in  wires  under  the  N  pole  is  directed  into  the 
paper,  or  away  from  the  observer.  Similarly,  the 
inductors  under  the  S  pole  must  urge  the  current 
"out"  or  toward  the  observer. 

Now  we  must  imagine  a  wire  across  the  back  of 
the  armature  which  carries  the  current  from  the  far 


ARMATURE    AND    FIELD    WINDINGS 


47 


end  of  inductor  No.  1  to  the  far  end  of  No.  2.  The 
pressure  generated  in  No.  2  then  assists  that  of  No  1, 
and  the  current  is  forced  along  the  wire  shown, 
across  the  front  end  of  the  armature,  to  inductor 
No.  3,  passing  a  commutator  bar  which  is  inactive 
because  out  of  contact  with  either  brush.  The  cur- 
rent flows  ''in''  along  No.  3,  across  the  back  of  the 
armature  to  No.  4,  across  the  front  to  No.  5,  then 
across  the  back  to  No.  6,  finally  reaching  bar  B  and 
leaving  the  armature.    Six  inductors  in  series  have 


r 

1 

/. 

I   m 

^^^^^^^^Ko^^^^^^^^ 

.......       -.. 

This  shows  how  the 
formed  coils  are  placed 
on  the  armature  and  the 
terminals  connected  to 
the  risers  from  the  com- 
mutator bars. 


added  their  voltages,  so  that  if  the  average  pressure 
induced  in  each  is  .9  v.,  the  machine  generates  5.4  v. 
The  other  inductors,  commencing  with  No.  7, 
and  working  through  Nos.  8,  9,  10,  11  and  12,  per- 
form a  service  exactly  similar  to  that  of  the  first 
set,  producing  a  voltage  of  5.4  in  parallel  or  multiple 
with  the  first  voltage  considered.  If  the  current  is 
5  amperes  in  each  wire,  the  total  current  sent  out 
of  Bar  B  is  10  amperes.  Thus  we  have  the  armature 
current  twice  that  in  one  inductor,  and  armature 
voltage  equal  to  the  product  of  half  the  inductors 
times  the  average  e.m.f .  in  one. 


48  APPLIED     ELECTRICITY 

Generators  with  4,  6  or  more  poles  are  not  un- 
common. The  armatures  may  be  so  wound  that 
there  are  only  2  parallel  paths  for  the  current  or 
so  that  there  are  as  many  paths  as  poles,  or  with 
other  numbers  dependent  upon  variations  in  the 
methods  of  connection.  With  the  ''lap"  or  "multi- 
ple" winding  there  are  at  least  as  many  paths  as 
poles,  and  there  must  be  as  many  brushes  as  poles. 
With  "series"  or  "wave"  winding  there  are,  in  gen- 
eral, only  two  parallel  paths,  and  there  may  be  either 
2  brushes  or  as  many  brushes  as  poles. 

There  are  often  more  than  2  inductors  in  one 
slot,  especially  in  small  machines.  This  necessitates 
cross  connections  on  the  front  end  of  the  armature 
in  addition  to  those  running  to  the  commutator 
bars.  Referring  to  Fig.  13,  the  wire  might  run  "in" 
along  the  top  of  slot  c,  then  across  the  back  and 
"out"  along  the  bottom  of  d,  then  across  to  slot  c 
again  (avoiding  the  commutator)  and  thus  through 
c  and  d  several  times.  The  coil  may  be  wound  on  a 
"form"  and  taped  and  varnished  before  being  put 
upon  the  armature,  and  this  is  the  usual  practice  for 
"multipolar"  machines  (having  4  or  more  poles). 

The  armature  of  Fig.  13  could  be  made  to  pro- 
duce alternating  current  by  taking  away  all  the  com- 
mutator bars  except  A  and  B  and  changing  each  of 
these  to  a  "slip  ring,"  so  that  each  would  be  continu- 
ously in  contact  with  the  same  brush. 


VIII 
LOSSES  AND  REACTIONS  IN  D.C.  GENERATORS 

Copper  Losses. — When  a  d.c.  generator  is  send- 
ing current  through  a  line  and  a  load,  as  in  Fig.  14, 
the  number  of  amperes  is  found  by  dividing  the  gen- 
erated voltage  by  the  total  ohms  of  the  circuit,  which 
means  the  sum  of  the  resistances  of  the  load  and 
the  line  plus  the  resistance  in  the  armature,  commu- 
tator and  brushes.    It  is  to  be  emphasized  that  the 


Fig.    14. — Separately    excited    d.c.    generator. 

electromotive  force  has  to  overcome  resistance  in  the 
generator  as  well  as  in  the  external  circuit.  Thus 
there  is  a  conversion  of  electric  energy  into  heat 
energy  in  the  inductors,  the  watts  wasted  here  being 
termed  the  "armature  copper  loss."  It  is  calculated 
by  multiplying  the  armature  resistance  by  the  square 
of  the  number  of  amperes  flowing  through  it ;  in  Fig. 
14,  this  is  the  same  as  the  line  current,  but  in  Fig. 
16  it  is  greater  by  the  amount  of  current  used  in  the 
shunt  field. 

One  can  measure  the  resistance  of  an  armature 
by  sending  a  small  current  through  it,  measuring 
the  voltage  drop,  and  calculating  by  Ohm's  law. 

49 


50 


APPLIED     ELECTRICITY 


The  resistance  of  an  armature  may  also  be  de- 
termined from  the  size  and  length  of  the  wire  used 
in  winding. 

As  shown  in  Fig.  15,  an  armature  coil  may  con- 
sist of  a  single  turn  of  wire,  running  from  one  com- 
mutator bar  to  and  through  the  top  of  one  slot,  then 
across  the  back  of  the  armature  and  forward  through 
the  bottom  of  another  slot  to  the  bar  adjacent  to 


Fig.  15. — "Multiple  Winding"  for  a  6  pole  d.c.  armature.  The  heavy 
radial  lines  with  arrow  heads  represent  the  inductors.  The  dotted  induc- 
tors are  in  reality  placed  beneath  the  solid  ones  shown  beside  them.  The 
brushes  are  drawn  inside  the  commutator  to  simplify  the  sketch. 


the  first  one.  The  radial  part  of  the  line  represents 
the  inductor,  which  is  shown  dotted  when  at  the  bot- 
tom of  a  slot. 

In  small  machines  there  are  usually  several 
turns  per  coil,  the  wire  running  around  several  times 
through  the  same  two  slots,  with  the  ends  connected 
to  adjacent  bars.  If  each  coil  in  Fig.  15  contained  30 
turns  of  No.  10  wire,  of  a  total  length  of  80  ft.,  its 
resistance  would  be  80  X  .000997  =  .08  ohm.  As 
there  are  24  coils  and  6  brushes,  there  are  four  coils 


LOSSES   AND   REACTIONS  51 

in  series,  with  a  resistance  of  4  X  -08  or  .32  ohm. 
Current  flows  through  the  armature  in  6  parallel 
paths,  and  hence  the  combined  resistance  =  .32  -^  6 
=  .053  ohm. 

The  "current  rating''  of  an  armature  is  the  num- 
ber of  amperes  it  can  safely  carry.  It  is  evidently 
equal  to  the  current  which  one  wire  can  carry  multi- 
plied by  the  number  of  parallel  branches  in  the 
armature. 

If  the  safe  current  in  each  inductor  of  the  arma- 
ture of  Fig.  15  is  15  amperes,  the  current  rating  is 
6  X  15,  or  90  amperes.  There  being  120  inductors 
in  series,  the  generated  voltage  =  120  X  -7  if  condi- 
tions of  flux  and  speed  are  so  arranged  as  to  give  .7 
volt  per  inductor.  The  power  generated  t=:  84  X  9^ 
=  7560  watts.  The  armature  copper  loss  =  90  X  90 
X  .053  =  430  watts,  so  that  the  output  of  the  arma- 
ture is  7130  watts. 

The  same  armature  might  have  a  "series"  or 
"wave"  winding,  which  has  only  two  parallel  paths. 
With  the  same  total  number  of  inductors  there  would 
be  360  in  series  and  a  voltage  three  times  as  high 
as  with  the  multiple  winding.  The  current  could, 
however,  be  only  30  amperes,  and  the  power  would 
be  the  same  as  before.  The  armature  copper  loss 
may  be  shown  to  be  the  same  with  both  methods  of 
winding. 

There  is  resistance  at  the  contact  of  the  brushes 
and  the  commutator  and  in  the  material  of  the 
brushes.  This  causes  an  electrical  loss  which  varies 
with  the  load  carried. 

Another  important  loss  of  power  occurs  in  the 
field  windings.  In  Fig.  14  suppose  the  field  resistance 
to  be  17  ohms  and  the  current  supplied  to  be  3  am- 
peres. The  power  loss  =  3  X  3  X  17  =  153  watts. 
If  the  rheostat  is  set  at  4  ohms,  there  is  a  further 
loss  there  of  36  watts.  Compound  generators  have 
copper  losses  in  both  shunt  and  series  fields.  Thus, 
if  the  "brush  voltage"  is  120  in  Fig.  16,  the  shunt 
current  8  amperes,  the  load  current  200  amperes  and 
the  series  field  resistance  .01  ohms,  the  shunt  loss  = 


I     •    a2  c 

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"^    «    j2    ft 


'lis 

2  fe  »^ 

h  a>  g  q; 


iJ  ft 

0^ 


^  a>  ft 


LOSSES   AND   REACTIONS  ^3 

120  X  8  =  960  watts    and    the    series    field    loss  = 
200  X  200  X  -01  =  400  watts. 

Mechanical  and  Iron  Losses. — In  a  d.c.  generator 
there  are  three  kinds  of  mechanical  power  losses  and 
two  different  iron  losses.  The  former  include  bear- 
ing friction,  "windage''  or  air  friction,  and  friction 


Fig.   16. — This  compound  wound  generator  has  a  loss  of  960  watts  in  the 
shunt  winding  and   400   watts   in  the  series   coils. 

between  the  stationary  brushes  and  the  moving  com- 
mutator. The  iron  losses  include  those  due  to  "hys- 
teresis" and  "eddy  currents.'' 

Armatures  are  influenced  by  the  poles  which 
surround  them  and  become  magnets  themselves.  As 
they  rotate  their  magnetic  condition  must  be  contin- 
ually changing,  for  each  particle  of  iron  is  magnet- 
ized as  it  passes  one  pole  and  remagnetized  in  the 
opposite  sense  when  it  reaches  the  next.  The  par- 
ticles oppose  this  change  by  what  seems  like  internal 
friction,  and  the  energy  thus  wasted  (changed  into 
heat)  is  the  "hysteresis  loss." 

As  the  lines  of  force  cut  through  the  armature 
iron,  currents  of  electricity  are  induced  in  it  by 
exactly  the  same  process  as  the  currents  in  the 
armature  wires.  These  "eddy  currents"  require  the 
expenditure  of  energy  and  produce  heat.  To  minim- 
ize the  currents,  which  tend  to  flow  parallel  to  the 
inductors,  the  armature  is  built  of  thin  circular 
sheets,  or  "laminations"  of  iron.  The  oxide  on  the 
faces  of  the  sheets  forms  an  insulator  which  pre- 


54  APPLIED     ELECTRICITY 

vents  passage  of  the  currents  and  thus  reduces  the 
eddy  loss  to  a  relatively  small  amount. 

Armature  Drop  and  Reaction. — In  a  separately 
excited  machine  (Fig.  14)  or  a  shunt  generator  the 
brush  voltage  is  lower  when  a  load  is  carried  than 
when  the  external  circuit  is  open.  One  cause  is  the 
"armature  drop/'  another  is  the  *'brush  contact 
drop"  and  a  third  is  the  "armature  reaction.''  It  is 
to  overcome  these  as  well  as  "line  drop"  that  the 
compound  winding  shown  in  Fig.  16  is  used. 

There  is  always  a  voltage  drop  in  a  conductor 
which  carries  current,  and  this  is  true  even  in  an 
armature  where  voltage  is  being  generated.  Then 
in  the  armature  of  Fig.  15  with  a  resistance  of  .053 
ohm,  the  drop  is  90  X  -053  =  4.77  volts  when  the 
machine  produces  90  amperes.  This  would  reduce 
the  voltage  from  the  generated  pressure  of  84  to 
79.23  volts. 

Due  to  the  contact  resistance  between  the  com- 
mutator and  brushes  there  will  be  a  further  drop 
of  about  one  volt. 

The  armature  inductors  carrying  current  tend 
to  make  the  armature  a  magnet  with  poles  between 
the  pole  pieces  of  the  generator.  This  results  in 
lessening  the  total  flux  through  the  armature  and 
makes  other  disturbances  which  lower  the  efficiency 
of  the  machine.  The  whole  effect,  known  as  "arma- 
ture reaction,"  varies  with  the  amount  of  current 
being  drawn  by  the  external  circuit.  Armature 
reaction  is  counteracted  in  various  ways,  such  as 
shifting  the  brushes,  building  the  generator  with 
"interpoles,"  and  putting  "compensating  windings" 
upon  the  pole  faces. 


IX 
ELECTROLYSIS 

The  word  "Electrolysis"  is  used  to  indicate  the 
carrying  of  an  electric  current  through  a  solution. 
It  is  found  that  absolutely  pure  water  is  an  almost 
perfect  insulator,  but  that  the  presence  of  an  appre- 
ciable amount  of  any  one  of  a  number  of  substances 
makes  the  liquid  a  fairly  good  conductor.  These 
soluble  substances  are  called  "electrolytes/'  and  they 


Fig.  17. — The  d.c.  generator  sends  current  through  the  liquid  from  the 
"anode"  (marked  "+"),  to  the  "cathode"  (" — ")  when  the  switch  is 
down.  If  the  plates  are  lead  and  the  electrolyte  sulphuric  acid,  a  battery 
is    thus    produced,    capable   of   ringing    the   bell    when    the    switch    is    up. 

include  acids,  salts,  and  "bases"  (or  alkalis).  Sugar 
is  not  an  electrolyte. 

The  current  is  carried  into  and  out  of  the  solu- 
tions generally  by  metal  "electrodes,"  the  one  where 
the  current  enters  the  liquid  being  called  the  positive 
or  "anode,"  and  the  other  the  negative  or  "cathode." 
When  the  electrodes  are  far  apart  the  current  must 
pass  through  a  long  body  of  liquid,  hence  meeting 


55 


56  APPLIED     ELECTRICITY 

high  resistance;  when  the  electrodes  are  large  the 
conducting  body  of  liquid  has  a  cross  section,  which 
lowers  the  resistance.  Hence  in  all  kinds  of  bat- 
teries, including  stoVage  cells  (* 'accumulators"),  the 
effort  is  made  to  have  the  electrodes  as  large  and  as 
near  together  as  possible,  for  the  current  must  flow 
between  them  and  resistance  causes  loss  of  energy. 

Chemical  Effects. — When  current  flows  through 
a  solution,  as  in  Fig.  17  when  the  switch  is  closed,  it 
tends  to  separate  and  release  at  the  electrodes  the 
components  of  the  electrolyte.  For  example,  sulphu- 
ric acid  is  a  union  of  hydrogen  with  the  ^'sulphate 


DDOH 


Z^Ql 


— ■    —    —    —  /»•</    C9<t*r0^  cab/*.. 

Fig.  18. — Current  returns  to  the  generator  by  the  rails  and  through  the 
earth.  Part  of  the  earth  current  is  diverted  to  the  lead  covered  cable 
and  electrolysis  destroys  the  sheath. 

ion'' ;  when  current  passes  through  dilute  sulphuric 
acid  the  hydrogen  is  found  to  collect  at  the  negative 
electrode  and  the  sulphate  is  set  free  at  the  anode; 
being  chemically  active  it  immediately  combines 
with  water  there,  and  liberates  some  oxygen.  This 
gas  may  appear  as  bubbles  upon  the  anode  or  it  may 
unite  chemically  with  the  material  of  the  anode ;  the 
hydrogen  at  the  cathode  may  appear  as  bubbles  or 
combine  chemically  with  the  material  of  that  elec- 
trode. 

In  weak  solutions  the  resistance  is  practically 
proportional  to  the  amount  of  electrolyte  present. 
This  fact  is  utilized  ingeniously  by  certain  California 
engineers  who  determine  by  a  single  test  the  salinity 
of  water  in  their  steam  boilers.  A  pair  of  electrodes 
fixed  at  a  certain  distance  apart  are  immersed  in  a 
sample  of  the  water  and  connected  to  a  known  volt- 
age through  a  mil-ammeter.  A  single  reading  thus 
determines  the  resistance,  from  which  is  known  the 
number  of  grains  of  salt  in  a  gallon  of  water.    Then 


ELECTROLYSIS 


57 


water  which  is  too  impure  is  blown  out  and  fresh 
water  substituted. 

Moist  earth  carries  current  readily  and  makes 
trouble  between  electric  railway  companies  and  the 
people  who  own  subterranean  piping  or  metal  cov- 


Salinity  tester  used  by  the  Pacific  Gas  &  Electric  Company.  A  bottle  of 
boiler  water  is  brought  up  around  either  pair  of  electrodes  and  the  mil- 
ammeter  reads  the  current.  The  instrument  is  also  calibrated  to  give  a 
direct  reading  of  the  amount  of  salt  present. 


ered  cables.  Where  a  current  leaves  a  metallic  con- 
ductor (which  is,  then,  the  anode)  the  metal  is  dis- 
solved and  removed. 

Storage  Batteries. — If  two  lead  plates  are  used 
as  electrodes  in  dilute  sulphuric  acid  and  a  direct 
current  sent  through  the  solution,  as  in  Fig.  17,  it  is 
noticed  that  the  arrangement  soon  becomes  a  bat- 
tery, capable  of  sending  electricity  through  a  wire, 
though  it  was  not  a  battery  before  the  "charging 


58  APPLIED-    ELECTRICITY 

current"  passed.  This  current  changes  the  positive 
electrode  to  "lead  peroxide"  by  giving  oxygen  to  it, 
while  the  cathode  remains  lead.  The  chemical  action 
of  the  acid  upon  the  lead  peroxide  and  the  lead  then 
causes  an  e.m.f .  to  be  set  up  tending  to  send  current 
out  from  the  peroxide  to  the  lead  plate;  the  electrode 
by  which  the  charging  current  entered  the  cell  is  the 


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"Do  It  Electrically"  is  the  motto  aboard  this  submarine.  Large  storage 
batteries  are  charged  by  gas  engines  when  the  boat  is  on  the  surface 
and  they  supply  power  for  propulsion,  lights,  cooking,  and  many  other 
purposes   when  the  boat  is   submerged. 

one  by  which  current  tends  to  go  out  when  the  cell 
is  discharged  by  throwing  the  switch  upward. 

The  chemical  action  during  discharge  is  as  fol- 
lows :  The  hydrogen  of  the  acid  goes  to  the  peroxide 
plate  and  takes  off  some  of  the  oxygen,  forming 
water,  which  dilutes  the  acid.  The  remaining  oxy- 
gen of  the  peroxide  plate  is  exchanged  for  sulphate 
ions,  so  that  the  plate  becomes  lead  sulphate.  Other 
sulphate  ions  go  to  the  lead  plate  and  combine  with 
it,  forming  lead  sulphate.  So  the  sulphuric  acid  be- 
comes weaker,  losing  hydrogen  and  sulphate  ions; 
both  plates  become  coated  with  lead  sulphate. 

Sending  a  charging  current  through  again  ex- 
actly reverses  this  action,  taking  sulphate  ions  off 
both  plates  and  taking  oxygen  from  the  water  to 


ELECTROLYSIS 


59 


make  the  positive  plate  into  lead  peroxide.    The  sul- 
phuric acid  is  increased  in  strength. 

These  lead  storage  cells  are  very  widely  used. 
They  are  built  commercially  with  the  active  materi- 
als (spongy  lead  and  lead  peroxide)  in  grooves  or 
holes  upon  a  lead  backing.  One  cell  generally  con- 
tains several  positive  plates  and  several  negative 
plates  similarly  interconnected,  the  two  kinds  being 


This  small  storage  battery  does  the  work  of  two  shelf-fuls  of  dry  cells. 
It  is  charged  for  a  few  minutes  each  hour  by  means  of  a  clock  operated 
connection  to  a  d.c.  circuit. 


interspaced  and  prevented  from  short-circuiting  by 
wood  and  rubber  separators.  A  number  of  these 
cells  connected  together  form  a  "battery.'' 

All  cells  of  the  same  kind  have  equal  voltage, 
the  size  and  number  of  plates  affecting  only  the  re- 
sistance and  the  ^'capacity"  of  the  cell — of  course  in 
a  large  cell  there  is  more  active  material  than  in  a 
small  one,  and  it  can  give  more  ampere-hours  before 
it  is  discharged. 

The  voltage  of  a  battery  equals  the  sum  of  the 
voltages  of  all  the  cells  connected  in  series;  if  sev- 
eral cells  are  connected  in  parallel  the  pressure  is  no 
higher  than  that  of  one  of  them.  The  terminal  volt- 
age of  a  discharging  battery  is  less  than  the  gen- 
erated voltage  when  current  is  flowing,  just  as  in  a 
generator.     The  current  must  pass  through  resist- 


ELECTROLYSIS  61 

ance  in  the  cells  themselves,  and  hence  there  is  a 
voltage  drop,  depending  on  the  current  and  the  re- 
sistance. Toward  the  end  of  the  discharge  the 
resistance  increases,  causing  the  terminal  voltage  to 
diminish  rapidly.  The  volts  per  cell,  when  discharg- 
ing, drop  from  about  2.1  to  1.8. 

The  Edison  Storage  Cell. — In  order  to  overcome 
some  of  the  undesirable  features  of  the  lead  battery, 
such  as  weight,  acid  fumes,  and  the  necessity  for 
constant  and  skilled  supervision,  Edison  invented  the 
nickel-iron  storage  cell  which  has  become  popular 
for  electric  automobiles  and  many  other  uses.  The 
positive  plate  contains  perforated  tubes  of  nickel  hy- 
drate while  the  negative  plate  contains  pockets  of 
iron  oxide.  The  electrolyte  is  a  solution  of  caustic 
potash  (chemically  pure  lye) ,  which  does  not  give  off 
fumes.  It  is  not  changed  in  the  processes  of  charg- 
ing and  discharging,  which  are  chemically  equivalent 
to  transferring  oxygen  from  one  plate  to  the  other. 

The  voltage  of  a  single  Edison  cell  is  about  1.3, 
while  the  lead  cell  has  about  2  volts.  The  pressure 
needed  to  charge  an  Edison  cell  is  about  1.7  volts; 
to  charge  a  lead  cell,  2.3  volts.  These  figures  are,  of 
course,  higher  than  the  counter  pressure  generated 
in  the  cells,  because  the  charging  voltage  has  to  over- 
come the  voltage  drop  due  to  cell  resistance  as  well 
as  the  voltage  of  the  cell  itself. 

Primary  Batteries. — Many  batteries  are  in  use 
in  which  the  material  of  one  plate  is  so  changed  and 
removed  by  the  chemical  reactions  which  cause  cur- 
rent to  flow  that  it  cannot  be  restored  by  forcing 
current  through  the  cell  in  the  reverse  direction. 
These  are  called  "primary  batteries"  in  distinction 
from  accumulators  which  are  sometimes  called  **sec- 
ondary  batteries."  On  account  of  its  convenience 
and  low  resistance  the  "dry  cell"  is  one  type  of  pri- 
mary battery  very  much  used,  and  there  are  a  num- 
ber of  different  kinds  of  wet  cell  in  use.  To  make 
a  battery  for  experimental  purposes  one  has  only 
to  put  two  pieces  of  metal  of  different  kinds  (or  a 
piece  of  carbon  and  metal)  into  a  solution  of  salt,  an 


62  APPLIED     ELECTRICITY 

acid  or  an  alkali.  A  dime  and  a  copper  cent  separated 
by  a  piece  of  moist  blotter  will  produce  enough  cur- 
rent to  make  a  click  in  a  telephone  receiver.  The 
action  of  such  a  battery  depends  upon  the  difference 
in  the  chemical  action  upon  the  two  electrodes. 
Since  any  electrolyte  acts  differently  upon  every  kind 
of  solid  conductor,  any  two  metals  (or  carbon  and 
one  metal)  will  serve  for  a  battery  with  any  electro- 
lyte. The  voltage  is  different  for  every  combination, 
varying  from  a  very  small  value  up  to  about  2  volts. 
The  "dry  cell"  which  has  sal  ammoniac  for  its  elec- 
trolyte (mixed  with  some  filler  to  a  sort  of  paste  and 
sealed  into  the  cell  with  wax)  generates  a  pressure 
of  1.5  volts. 


ELECTRIC   MOTORS 


Theory  of  Motor  Action. — Oersted  in  1819,  laid 
the  foundation  of  the  modem  electric  motor  when 
he  discovered  that  current  in  a  wire  affected  a  nearby 
compass  needle.  He  showed  that  the  electricity 
tends  to  cause  the  movement  of  a  magnet  pole  in  a 
circle  around  the  conductor.  In  Fig.  19,  for  instance, 
the  current  in  wire  A  exerts  a  force  upon  the  nearby 
north  pole,  trying  to  make  it  move  upward,  then  to 
the  right,  then  downward.  A  south  pole  wo|ild  tend 
to  circle  the  wire  in  the  opposite  direction. 


Fig.   19.- 


-Current  in  Wire  A  tends  to  force  the  north   pole  upward, 
reaction    drives    the    wire    itself    downward. 


The 


Direct  current  motors  are  built  with  stationary 
poles,  so  that  the  wires  themselves  are  made  to 
move,  just  as  a  man  trying  to  push  a  piano  across  a 
room  may  drive  himself  backward.  Then  in  Fig.  19, 
wire  A  moves  downward  and  the  wire  adjacent  to  a 

63 


64 


APPLIED     ELECTRICITY 


south  pole  moves  upward.  One  can  always  determine 
the  direction  by  remembering  the  old  Right  Hand 
Rule:  Grasp  the  wire  with  the  thumb  pointing 
along  it  in  the  direction  of  current  flow ;  then  a  north 
pole  will  follow  the  fingers  around  the  wire. 

The  armature  of  a  motor  is  built  of  laminated 
iron  with  slots  for  the  conductors.  (See  Fig.  20.)   The 


Fig.  20.— A  motor  armature.  The  currents  in  the  wires  may  be  thought  of 
as  producing  poles  in  the  armature  iron  which  are  attracted  and  repelled 
by  the  field  poles. 


commutator  of  a  d.c.  machine  acts  to  keep  the  cur- 
rent always  flowing  in  the  same  direction  in  the 
vicinity  of  any  pole.  In  Fig.  20  it  flows  "out"  (toward 
the  observer)  beside  the  north  pole  as  indicated  by 
the  dots  (arrow  points) ;  it  flows  "in"  (into  the 
paper)  beside  the  south  pole  as  shown  by  the  (+) 
marks  (arrow  feathers). 

If  an  armature  such  as  that  in  Fig.  19  is  supplied 
with  an  alternating  current  which  reverses  exactly 
**in  step''  with  the  rotation,  so  that  current  flows 
"in"  along  the  other  wire  when  that  is  beside  the 
north  pole,  the  machine  will  run  and  carry  a  load. 
This  is  the  principle  of  the  ^'synchronous  motor." 
It  follows  that  any  electric  generator  will  operate 
as  a  motor  if  supplied  with  current  under  proper 
conditions. 


ELECTRIC    MOTORS 


65 


Another  way  to  study  the  action  of  a  motor  is 
to  think  of  the  armature  as  an  electromagnet.  The 
coils  in  Fig.  20  tend  to  produce  a  north  pole  at  the 
top  of  the  armature  and  a  south  pole  below.  These 
are  pulled  and  pushed  by  the  field  magnets  according 
to  the  rule :  Unlike  poles  attract  and  like  poles  repel 
each  other.  -As  the  armature  rotates,  its  poles  con- 
tinually shift  through  the  metal  and  keep  in  their 


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Direct  current  motors  of  15  hp.  each  drive  these  deck  winches.  What  is 
the  current  input  to  one  of  the  116-volt  motors  when  a  load  of  25000  lbs. 
is  being  hoisted  3  ft.  per  sec.  if  the  overall  efficiency  of  the  machinery  is 
55%? 


positions  at  the  top  and  bottom,  because  the  wires 
at  the  right  are  always  carrying  current  in  while 
those  at  the  left  carry  it  out. 

Counter  Electromotive  Force.  —  In  a  rotating 
motor  armature  the  wires  are  passing  at  high  speed 
through  lines  of  force.  This  must  set  up  a  voltage  in 
these  wires,  whether  they  carry  current  or  not,  for 
an  e.m.f.  is  always  induced  in  a  conductor  being  cut 
by  lines  of  force.  This  voltage  may  either  help  or 
hinder  the  current  flow;  if  it  helps,  there  will  be 


66  APPLIED     ELECTRICITY 

more  current  and  more  magnetic  force  the  faster  the 
armature  goes,  which  is  contrary  to  reason  and  ex- 
perience. We  conclude  that  the  induced  e.m.f.  op- 
poses the  current  and  the  voltage  driving  it,  and 
name  it  "counter  e.m.f/'  or  "back  e.m.f." 

The  number  of  amperes  through  the  armature 
depends,  then,  upon  three  things:  the  armature  re- 
sistance, the  applied  voltage,  and  the  back  e.m.f. 
If  we  had  a  battery  circuit  of  2  ohms  total  resistance 
with  6  dry  cells  in  series,  the  voltage  would  be 
6  X  1.5  =^  9  volts,  and  the  current  4.5  amperes.  If 
one  of  the  cells  were  connected  backward,  however, 
the  effective  voltage  would  be  7.5  — 1.5  or  6  volts, 
and  the  current  3  amperes.  With  two  cells  connected 
backward  we  should  obtain  1.5  amperes.  If  three 
cells  were  opposing  the  rest  there  would  be  no  cur- 
rent, for  the  net  voltage  would  be  zero. 

If  the  counter  voltage  of  a  motor  equaled  the 
applied  pressure,  no  current  could  flow,  and  no  power 
would  be  drawn  from  the  line.  Hence  the  back  e.m.f. 
is  always  less  than  the  applied  voltage,  for  it  takes 
power  to  run  a  motor,  even  when  unloaded.  The 
armature  current  =  (applied  volts  —  back  e.m.f.)  h- 
armature  ohms. 

Motor  Starting.  —  When  an  armature  is  sta- 
tionary it  cannot  produce  any  counter  e.m.f.  If  the 
full  voltage  were  applied  a  very  heavy  current  would 
flow,  and  for  the  protection  of  the  windings  it  is 
customary  to  provide  some  sort  of  starting  appa- 
ratus which  limits  the  current  to  a  safe  value.  Direct 
current  shunt  motors  (which  are  wound  the  same  as 
shunt  generators)  are  started  with  a  variable  re- 
sistance in  series  with  the  armature,  as  shown  in 
Fig.  21.  By  means  of  a  sliding  contact  the  resistance 
is  lowered  step  by  step  as  the  motor  gains  speed,  and 
it  is  all  out  when  full  speed  is  nearly  reached. 

Alternating  current  motors  are  often  started  by 
applying  pressure  less  than  the  normal  line  voltage. 
"Compensators''  which  are  "step  down  transform- 
ers'' are  connected  to  the  line  and  supply  current  at 


ELECTRIC    MOTORS 


67 


low  voltage  during  the  starting  period.  In  other 
cases  resistance  is  introduced  into  some  of  the  motor 
circuits,  while  sometimes  the  connections  of  the  coils 
are  changed  temporarily  to  produce  effects  equivalent 
to  changing  from  parallel  to  series  circuits. 


Fig.  21. — starting  box  for  d.c.  shunt  motor.  To  protect  the  armature 
winding,  when  it  is  producing  little  or  no  back  electromotive  force,  the 
line  current  is  sent  through  a  series  resistance  which  is  decreased  as  the 
motor  speeds  up. 


Alternating  Current  Motors. — Synchronous  mo- 
tors are  used  when  it  is  desired  to  operate  machinery 
at  a  speed  exactly  proportional  to  that  of  the  gen- 
erators supplying  the  current. 

For  driving  the  d.c.  generators  for  electric  rail- 
ways and  in  other  applications  requiring  large 
amounts  of  power  it  is  usual  to  install  synchronous 
motors  in  preference  to  other  tjrpes,  largely  on  ac- 
count of  their  beneficial  effect  on  the  power  factor 
of  the  transmission  line. 

Since  they  can  not  carry  a  load  except  when 
exactly  in  step  with  the  alternations  of  the  current 
supply,  it  is  necessary  to  bring  them  up  to  speed  by 
some  special  device.  Sometimes  the  d.c.  generator 
is  connected  to  the  direct  current  line  and  run  as  a 
motor  during  the  starting  period;  in  other  installa- 


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ELECTRIC    MOTORS  69 

tions  the  synchronous  motor  is  started  without  load 
by  means  of  a  small  a.c.  motor  of  another  kind. 

For  all  ordinary  applications,  the  * 'induction 
motor''  is  most  widely  used.  The  construction  is 
simple,  cheap  and  rugged,  and  operation  and  mainte- 
nance are  easy  and  inexpensive.    The  tj^ical  motor 


There  are  far  less  "kicks"  in  the  mines  since  the  electric  motor  displaced 
the  time-honored  mule  locomotive.  In  some  mines  trolley  lines  are  used ; 
in  others  the  motors  are  driven  by  storage  batteries  carried  by  the  locomo- 
tives themselves   (as  in  this  illustration). 

of  this  type  has  no  sliding  contacts  and  only  the 
simplest  elements  of  a  winding  upon  the  revolving 
part  or  *'rotor." 

Alternating  currents  in  coils  wound  about  por- 
tions of  the  iron  "stator''  (stationary  part)  of  an 
induction  motor  produce  magnetic  poles  which  are 
north  when  the  current  flows  one  way  and  south 
when  it  reverses.  Other  poles  of  opposite  polarity 
are  produced  between  these  by  coils  wound  in  the 
opposite  direction.  The  result  is  that  north  poles  ap- 
pear at  several  points  around  the  stator  and  shortly 
after  (when  the  current  has  reversed)  they  appear 
in  different  places.  By  the  use  of  two  or  more  alter- 
nating currents  which  reverse  at  different  times 
there  is  produced  a  smooth  progression  of  the  poles 


70  APPLIED     ELECTRICITY 

around  the  inside  of  the  stator,  and  this  is  known  as 
the  "revolving  field." 

Just  as  in  the  alternating  generator,  the  wires 
near  the  poles  of  a  revolving  field  are  cut  by  the 
moving  lines  of  force  and  have  e.m.f.  induced  in 
them.  Thus  the  conductors  of  the  rotor,  which  are 
short  circuited  upon  each  other,  carry  heavy  induced 
currents,  but  no  electrical  contact  with  the  supply 
circuit  is  needed.  The  reaction  between  these  in- 
duced currents  and  the  magnet  poles  of  the  stator 
causes  the  rotor  to  turn.  As  it  gains  speed  it  almost 
catches  up  with  the  revolving  field  but  it  never  runs 
quite  as  fast.  If  it  did,  there  would  be  no  more 
induction  of  e.m.f.  on  the  rotor,  for  each  wire  would 
keep  beside  some  pole  and  there  could  be  no  cutting 
of  the  inductors  by  lines  of  force.  The  difference 
between  the  speeds  of  the  rotor  and  the  revolving 
field  is  known  as  the  '*slip,"  and  this  varies  whenever 
the  load  is  changed. 


XI 


MOTOR  CHARACTERISTICS 

Direct  Current  Motors. — For  operation  upon  d.c. 
circuits  motors  are  built  of  three  different  types. 
They  are  known  as  series,  shunt  and  compound 
motors,  the  names  referring  to  t>.e  connection  be- 
tween the  armature  and  the  field  winding,  as  in  the 


A  Westinghouse  motor-generator  set  run  by  direct  current  and  producing 
alternating  current.  This  last  is  raised  to  high  voltage  by  means  of 
transformers  and  then  "rectified"  for  use  in  precipitating  dust  in  flue  gases. 


case  of  d.c.  generators.  The  magnet  coils  in  a  series 
motor  consist  of  a  few  turns  of  large  wire  carrying 
the  whole  current  of  the  machine.  A  shunt  motor 
has  field  windings  of  small  wire  and  many  turns, 
connected  in  multiple  or  shunt  with  the  armature. 
Both  series  and  shunt  coils  are  put  upon  the  field  of 
a  compound  motor. 

71 


72  APPLIED     ELECTRICITY 

The  "torque"  or  turning  effort  of  a  motor  de- 
pends upon  the  strength  of  the  magnetic  field  and 
hence  is  proportional  to  the  field  current  except  for 
the  disturbing  effects  of  saturation  and  armature 
reaction.  Therefore,  decreasing  the  field  Current  to 
half  value  cuts  the  torque  approximately  in  two. 


This  direct  current  G-E  fan  runs  at  1500  r.p.m.  When  the  fan  blades 
are  removed,  the  motor  runs  at  about  3500  r.p.m.  Is  the  field  shunt.  sfs-ifsL 
or  compound   wound? 

This  is  on  the  assumption  that  the  current  in  the 
armature  remains  constant,  for  the  torque  is  directly 
proportional  also  to  armature  amperes.  Then  in  a 
series  motor,  where  the  same  current  flows  through 
armature  and  field,  triple  current  would  give  nine 
times  the  torque  were  it  not  for  the  disturbing  fac- 
tors mentioned  above.  (On  test  a  certain  500-volt 
machine  was  found  to  give  with  60  amperes  five 
times  as  great  a  torque  as  with  20.) 

Series  motors  are  used  for  electric  railways, 
hoists,  cranes,  etc.  When  a  street  car  starts  up-hill 
from  the  level  the  motors  at  once  lose  speed  and  the 
counter  e.m.f.  decreases.  More  current  is  thus  per- 
mitted to  flow  and  this  raises  the  torque  of  the 
motors  sufficiently  to  carry  the  increased  load.  This 
flexibility  in  regard  to  torque  and  speed  is  what 
makes  the  series  d.c.  motor  the  most  convenient  for 
all  such  applications  as  hoisting  and  traction  where 
heavy  and  variable  loads  must  be  frequently  started 
and  stopped.  It  is  necessary  to  keep  a  series  motor 
coupled  to  its  load,  however,  and  to  control  it  care- 


MOTOR    CHARACTERISTICS  73 

fully,  for  if  the  load  is  removed  the  speed  will  run 
very  high,  possibly  ruining  the  armature  by  cen- 
trifugal action. 

Shunt  Motors. — When  a  shunt  motor  is  operated 
without  a  load  it  does  not  run  faster  than  a  certain 
speed,  behaving  like  a  steam  engine  with  a  governor, 
in  contrast  to  the  series  motor  which  acts  like  an  un- 
governed  automobile  engine.  At  the  "no  load  speed" 
of  a  shunt  motor  it  develops  a  back  e.m.f.  almost 
equal  to  the  applied  voltage,  so  that  only  a  small 
current  can  flow.  A  slight  increase  in  the  speed  would 
raise  the  back  e.m.f.  so  high  that  no  current  could 
enter  the  armature  and  the  machine  could  take  no 
power  from  the  line. 

When  a  shunt  motor  is  required  to  drive  a  load 
it  must  absorb  more  watts  than  when  running  idle. 
Suppose  100  volts  applied  to  the  armature  and  the 
back  e.m.f.  at  no  load  equals  99.  The  net  voltage 
driving  current  through  the  armature  is,  then,  1  volt, 
and  the  current  will  be  10  amperes  if  the  resistance 
is  0.1  ohm.  If  a  load  is  then  applied  which  requires 
an  input  of  4  kw.,  the  current  must  rise  to  4000  -f- 
100,  or  40  amperes,  which  means  that  the  net  voltage 
must  be  40  X  0.1  =  4.  The  back  e.m.f.  must  drop  to 
96  volts,  which  means  that  the  speed  must  drop  to 
96/99  of  the  no  load  speed.  At  full  load  a  shunt 
motor  runs  about  5%  slower  than  at  no  load. 

It  is  often  desirable  to  run  a  shunt  motor  at 
other  than  normal  speed.  In  a  machine  shop,  for 
instance,  a  motor  driven  lathe  should  have  several 
available  speeds  suitable  for  different  jobs.  There 
are  four  ways  of  accomplishing  this:  (1)  By  install- 
ing a  multivoltage  system;  (2)  by  rheostat  control 
of  armature  current;  (3)  by  changing  the  field  flux 
mechanically;  (4)  by  rheostat  control  of  the  field 
current. 

With  two  or  more  generators  one  can  apply  to 
the  motor  armature  different  voltages  and  obtain 
speeds  in  proportion.  The  field  strength  may  be  kept 
constant  by  using  always  the  same  voltage  for  ex- 


74 


APPLIED     ELECTRICITY 


citation.  A  similar  effect  may  be  obtained  by  put- 
ting a  rheostat  in  series  with  the  armature  and  thus 
lowering  the  applied  e.m.f .  by  means  of  the  voltage 
drop.  This  method  is  wasteful  of  power,  while  the 
first  is  expensive  and  complicated. 

Weakening  the  field  of  a  shunt  motor  by  moving 
the  poles  and  armature  farther  apart  or  by  putting 


Fig.  22. — Cutting  down  the  field  current  in  this  shunt  motor  by  means  of 
the  rheostat  causes  the  armature  to  run  faster.  It  must  do  this  to  keep 
the  counter  electromotive  force  nearly  equal  to  the  voltage  of  the  line. 

resistance  into  the  exciting  circuit  changes  the  speed 
(see  Fig.  22).  Suppose  the  unloaded  100-volt  motor 
considered  above  had  its  field  weakened  7%  — what 
alteration  in  speed  would  occur?  The  back  e.m.f. 
would  instantly  fall  to  about  92  volts  and  a  large 
current  would  flow  (8  -^  0.1  =  80  amperes) .  This 
would  produce  a  strong  torque  and  speed  up  the 
armature  until  the  back  e.m.f.  reached  approxi- 
mately^ its  former  value.  Weakening  the  field 
increases  the  speed. 

Compound  Wound  Motors. — Some  motors  have 
a  series  winding  in  addition  to  the  shunt  coils  on  the 
field  poles.  Imagine  current  supplied  to  a  compound 
generator,  entering  at  the  positive  (+)  terminal 
(from  which  the  current  was  sent  out  when  the 
machine  was  generating) .  Would  the  series  field  help 
or  oppose  the  shunt  field,  and  which  way  would  the 
armature  rotate  ?  The  current  would  flow  in  the  old 
direction  through  the  shunt  coil  (from  the  +  to 
the  —  brush),  but  in  the  reverse  direction  through 
the  series  coil.  Hence  the  field  will  be  weakened  by 
the  series  coil.    The  armature  will  rotate  in  the  old 


MOTOR    CHARACTERISTICS  75 

direction,  for  it  must  produce  an  induced  e.m.f. 
opposing  current,  and  hence  directed  out  at  the  + 
brush.  When  a  load  is  put  upon  this  motor  the  in- 
creased current  in  the  series  coil  tends  to  weaken 
the  field  and  hence  to  increase  the  speed.  Thus  a 
compound  motor  can  be  arranged  to  have  a  constant 
speed  with  varying  load. 

A  "cumulative  compound  winding"  is  produced 
when  the  series  coil  is  connected  the  opposite  way, 
so  as  to  assist  the  shunt  coil.  Such  a  winding  gives 
a  strong  torque  at  starting,  due  to  the  heavy  series 
current  and  the  strong  field  it  produces.  An  increase 
of  load  in  such  a  machine  causes  the  speed  to  drop 
more  than  with  a  simple  shunt  winding,  as  the  in- 
crease of  current  strengthens  the  field.  Such  a  char- 
acteristic is  desired  for  such  machines  as  punch 
presses,  shears,  etc.  These  motors  with  various  de- 
grees of  compounding  are  used  also  for  elevators, 
rolling  mill  machinery,  etc. 

Alternating  Current  Motors.  —  A  synchronous 
motor  runs  at  a  constant  speed  which  is  determined 
by  the  number  of  poles  and  the  frequency  with  which 
the  supply  current  reverses  its  direction.  Adding  or 
taking  off  the  load  changes  the  number  of  amperes, 
and  varying  the  field  strength  changes  the  "phase 
relation"  of  the  supply  current,  but  the  motor  runs 
at  constant  speed  unless  the  load  is  heavy  enough  to 
make  it  "fall  out  of  step"  and  come  to  a  standstill. 

Induction  motors  without  load  run  at  nearly 
synchronous  speed.  This  can  be  calculated  from  the 
number  of  poles  and  the  frequency  of  the  supply  cir- 
cuit. For  "60  cycle  current"  the  revolutions  per 
second  =  60  -^  no.  of  pairs  of  poles.  Thus  a  6  pole 
motor  makes  20  revolutions  per  second  or  1200  r.p.m. 
unloaded.  The  50  cj^cle  current  used  in  Southern 
California  drives  a  4  pole  synchronous  motor  at  1500 
r.p.m. 

As  the  load  on  an  induction  motor  is  increased 
its  speed  decreases,  the  "slip"  varying  from  prac- 
tically zero  to  5%  or  more.    Some  motors,  specially 


MOTOR    CHARACTERISTICS 


77 


built  with  high  resistance  rotors,  have  a  slip  of  10  or 
15%.  Such  machines  are  used  for  driving  the  rolls 
in  steel  mills  and  similar  work  where  the  load  is 
heavy  and  intermittent.  With  a  punch  press,  for 
instance,  such  a  motor  can  speed  up  and  deliver 
energy  to  a  heavy  flywheel  during  the  interval  be- 
tween operations,  and  then  slow  down  to  give  the 


An  induction  motor  driving  a  wood  saw.  This  small  machine  runs  at 
1200  r.p.m.  when  unloaded,  and  is  rated  at  5  hp.  Calculate  the  number  of 
poles  and   the   frequency   of  the   alternating   current   supply. 


flywheel  a  chance  to  do  much  of  the  work  of  driving 
the  punch.  A  constant  speed  motor  would  be  of  very 
little  value  for  such  applications  for  it  would  be  very 
heavily  overloaded  part  of  the  time  and  idle  for  the 
remainder.  The  cumulative  compound  d.c.  motor  and 
the  induction  motor  with  large  slip  are  much  used 
with  flywheels  for  this  class  of  work. 


XII 

ELECTRIC  METERS 

Direct  Current  Instruments.  —  Practically  all 
electrical  meters  operate  by  reason  of  the  production 
of  magnetic  fields  by  electric  currents.  The  earliest 
indicating  instrument  was  merely  a  single  wire  held 
above  a  compass  needle.  A  flow  of  electricity  in  the 
wire  caused  the  needle  to  turn  through  an  angle  de- 
pendent upon  the  strength  of  the  current.  Running 
the  wire  below  the  needle  doubled  the  turning 
moment  and  it  was  a  short  step  to  the  simple  gal- 
vanometer which  consisted  of  a  compass  mounted  in 
a  coil  of  wire  with  the  needle  perpendicular  to  the 
axis  of  the  coil. 

There  are  serious  disadvantages  connected  with 
the  use  of  the  "moving  needle"  type  of  instrument, 
and  these  are  eliminated  in  the  "moving  coil"  and 
"magnetic  vane"  meters  now  commonly  used.  The 
moving  coil  meter  of  the  D'Arsonval  type  has  a  coil 
like  the  armature  winding  of  a  motor,  and  this  is 
placed  in  a  magnetic  field.  When  current  (which  is 
led  into  and  out  of  the  coil  through  spiral  springs 
around  the  shaft)  flows  through  the  winding,  the 
armature  turns  for  the  same  reason  that  a  motor 
revolves,  but  the  springs  restrain  the  motion  so  that 
the  attached  pointer  moves  only  a  limited  distance. 
The  torque  depends  upon  the  armature  current,  and 
hence  the  pointer  indicates  upon  its  scale  a  reading 
proportional  to  the  current.  (See  Fig.  6,  p.  23.) 

In  instruments  of  this  type  every  motion  of  the 
coil  causes  the  metal  bobbin  upon  which  it  is  wound 
to  move  through  a  strong  magnetic  field.    This  in- 

78 


ELECTRIC    METERS  79 

duces  eddy  currents  in  the  metal,  the  effect  of  which 
is  to  slow  the  motion  and  prevent  vibration  of  the 
coil  after  the  needle  reaches  the  point  where  the 
reading  should  be  made.  Such  instruments  are  called 
"dead  beat/'  Other  meters  do  not  depend  upon  eddy- 
current  "damping"  but  contain  air  chambers  in  which 
move  vanes  which  stop  vibration  by  air  friction. 


ZenoAdjuster 


Edgewise  Ammeter.     This  instrument  is  used  on  station  switchboards  for 
metering   direct    current. 


Light  moving  coils  with  delicate  springs  can  not 
carry  heavy  currents,  and  ammeters  of  this  type 
usually  contain  "shunts'*  which  carry  a  definite  frac- 
tion of  the  total  amperes  in  multiple  with  the  coil. 
The  scale  of  the  instrument  is  marked  or  "cali- 
brated" to  indicate  the  total  current  flowing  in  the 
coil  and  shunt. 

A  "milliammeter"  or  "mil-ammeter"  is  adapted 
to  measure  small  currents  and  marked  in  thou- 
sandths of  an  ampere.  Suppose  the  coil  of  such  an 
instrument  to  have  a  resistance  of  9  ohms  and  imag- 
ine a  coil  of  991  ohms  connected  in  series  with  it 
inside  the  case.  There  would  be  a  total  of  1000  ohms 
between  terminals,  and  if  a  pressure  of  30  volts  were 
applied,  the  current  flow  would  be  only  0.030  am- 
peres or  30  mil-amperes.  The  scale  reading  would 
be  30,  exactly  equal  to  the  voltage.  A  voltmeter, 
then,  is  simply  a  very  sensitive  galvanometer  or  am- 
meter with  large  resistance.  Such  an  instrument  is 
calibrated  by  connecting  it  in  multiple  with  a  stand- 


80  APPLIED     ELECTRICITY 

ard  voltmeter  and  altering  the  voltage  by  suitable 
steps. 

If  it  is  desired  to  use  a  150-volt  voltmeter  on  a 
1200-volt  circuit  it  is  necessary  to  put  more  resist- 
ance in  series  with  it.  Manufacturers  supply  "multi- 
pliers" which  are  simply  resistance  coils  to  be  used 
in  this  way.  The  meter  reading  must  be  multiplied 
by  the  appropriate  factor  to  get  the  pressure. 


Multiplier  to  use  in  connection  with  a  voltmeter  or  wattmeter.  What 
resistance  should  it  have  to  make  a  9000-ohm  voltmeter  read  140  on  a  560- 
volt  circuit?     What  would  the  multiplier  be  called? 

Electrodynamometer  Instruments. — ^Many  mov- 
ing coil  ammeters  and  voltmeters  have  magnetic 
fields  produced  either  by  permanent  magnets  or  by 
electro-magnets  excited  by  a  steady  current.  The 
"electro-dynamometer''  ammeter  or  voltmeter,  how- 
ever, has  a  field  coil  connected  in  series  with  the 
moving  coil,  so  that  the  torque  depends  upon  the 
square  of  the  current.  The  instruments  contain  no 
iron,  and  the  reaction  between  the  coils  is  spoken  of 
as  electro-dynamic,  rather  than  electro-magnetic. 
The  reading  scale  of  such  an  instrument  is  not  regu- 
lar or  uniform  but  the  marks  are  much  wider  apart 
at  some  places  than  others. 

The  power  in  a  d.c.  circuit  equals  the  product 
of  volts  times  amperes,  and  it  can  be  conveniently 
metered  by  an  electro-dynamometer  instrument.  The 
moving  coil  (with  high  resistance)  is  connected 
across  the  line  and  takes  a  current  proportional  to  the 
voltage.  The  stationary  coil  is  put  in  series  with 
the  load  and  so  carries  the  load  current.    The  torque 


ELECTRIC    METERS  81 

developed  is  proportional  to  the  product  of  pressure 
and  current  and  hence  the  scale  may  be  calibrated  in 
watts  or  kilowatts.  The  divisions  may  be  very 
nearly  uniform.  Such  a  wattmeter  may  be  used  for 
high  voltages  by  connecting  a  multiplier  in  series 
with  the  voltage  coil,  and  for  large  currents  with  the 
help  of  shunts  like  ammeter  shunts. 


General  Electric  Wattmeter.  Why  are  there  more  terminals  than  on  a 
voltmeter  ?  Is  this  instrument  built  for  a  switchboard  or  for  occasional  use  ? 
For  connections,   see  Fig.   5— ^page  15. 

Metering  Alternating  Currents. — If  the  current 
is  reversed  in  a  moving  needle  instrument  or  in  one 
having  a  moving  coil  and  a  permanent  magnet,  the 
pointer  will  be  seen  to  deflect  in  the  opposite  direc- 
tion. Such  meters  can  not,  then,  be  used  on  alternat- 
ing current  circuits.  Electro-dynamometer  instru- 
ments, however,  operate  perfectly  with  alternating 
current,  for  the  field  of  the  stationary  coil  reverses 
as  often  as  the  current  in  the  moving  coil,  thus  pro- 
ducing torque  always  in  the  same  direction.  Hence 
electrodynamometers  are  often  used  for  a.c.  circuits, 
as  ammeters,  voltmeters  and  wattmeters.  They  may 
be  calibrated  with  direct  current  and  used  on  either 
kind  of  circuit. 

Various  other  meters  have  been  developed  for 
use  with  both  direct  and  alternating  current.  The 
"electrostatic  voltmeter"  consists  of  moving  and 
stationary  vanes  which  are  charged  with  static  elec- 
tricity by  connecting  to  the  opposite  sides  of  a  high 


82  APPLIED     ELECTRICITY 

potential  circuit.  The  vanes  are  drawn  toward  each 
other,  moving  a  pointer  against  the  restraint  of  a 
spring.  A  fountain  pen  rubbed  upon  a  coat  sleeve 
will  attract  bits  of  paper  by  a  similar  electrostatic 
action. 

A  device  used  for  both  ammeters  and  voltmeters 
is  the  "hot  wire."  Current  through  a  piece  of  re- 
sistance wire  heats  it,  causing  expansion  which  per- 
mits a  spring  to  pull  a  pointer  across  a  scale.  Another 
scheme  is  to  use  for  a  voltmeter  or  ammeter  a  sta- 
tionary coil  surrounding  two  light  parallel  iron  rods, 
one  of  which  is  fixed  in  position.  The  other  is 
attached  to  the  pointer  and  can  move  around  the 
inside  of  the  coil,  always  keeping  parallel  to  the  fixed 
rod.  Both  rods  are  magnetized  when  current  flows, 
and  the  repulsion  of  like  poles  causes  one  to  move 
away  from  the  other.  Still  another  device  consists 
of  a  soft  iron  plunger  which  is  sucked  into  a  coil 
when  current  flows  around  it,  the  plunger  being  sup- 
ported by  the  spindle  which  carries  the  pointer. 
Pocket  instruments  for  testing  dry  cells  are  of  this 
type.  All  these  meters  will  work  with  more  or  less 
accuracy  upon  alternating  as  well  as  direct  current 
circuits,  for,  obviously,  reversing  the  current  does 
not  reverse  the  effect  upon  the  pointer. 

Many  alternating  current  voltmeters,  ammeters 
and  wattmeters  are  of  the  "induction  type."  In 
these  a  rotating  field  is  produced,  as  in  the  induction 
motor,  and  this  induces  in  the  rotor  short  circuit 
currents  which  tend  to  turn  it  on  its  axis.  A  re- 
straining spring  and  a  pointer  complete  the  moving 
element.  Such  instruments  can,  of  course,  only  be 
used  on  a.c.  circuits.  An  induction  wattmeter  has 
certain  coils  connected  across  the  line  and  others  in 
series  with  the  load;  ammeters  and  voltmeters  have 
all  their  coils  in  series. 

Watthour  Meters. — Instruments  for  measuring 
energy  consumption  are  often  mistakenly  called 
"wattmeters."  A  watthour  meter  is  a  small  electric 
motor  so  constructed  as  to  use  up  very  little  en- 
ergy and  yet  to  run  at  a  speed  proportional  at  all 


ELECTRIC    METERS 


83 


times  to  the  power  taken  by  the  electrical  load  on 
the  line.  By  means  of  a  revolution  counter  a  record 
is  made  on  the  dial  of  the  number  of  revolutions 
of    the  armature,  thus  accounting  for  the  kw-hr. 


Interior  of   a   direct   current   watthour   meter.      Note   the    four   permanent 
horseshoe  magnets   and  the   retarding  disc  between  their   poles. 

that  have  passed  the  meter.  Many  d.c.  watt- 
hour  meters  have  commutators  and  brushes,  the 
armatures  being  of  high  resistance  and  connected 
across  the  line  so  as  to  carry  current  proportional 
to  line  voltage.  The  field  is  then  connected  in  series 
with  the  load.  Such  meters  have  no  iron  at  all  in 
the  magnetic  circuit,  which  means  that  the  flux  and 
torque  are  proportional  to  the  voltage  and  current 
and  no  complications  are  caused  by  variations  in 
permeability,  etc. 

Reversing  the  current  in  both  the  armature  and 
series  coils  of  such  a  meter  gives  torque  in  the  pre- 
vious direction,  and  hence  it  may  be  used  on  a.c. 
circuits.     For  several    reasons,    however,  watthour 


84 


APPLIED     ELECTRICITY 


meters  of  the  induction  type  are  generally  preferred 
for  such  service.  These  are  simply  induction  motors, 
lacking  commutator  and  brushes,  and  thus  having  no 
moving  contacts. 

It  is  necessary  in  all  watthour  meters  to  restrain 
the  motion  of  the  armature  or  else  even  a  light  load 
would  cause  rapid  rotation  and  high  readings  on  the 


Testing  meters  taken  from 
residences  in  Fresno,  Cal- 
ifornia. Are  these  watt- 
meters or  energy  meters? 


dials.  Usually  a  disc  of  aluminum  is  attached  to  the 
armature  shaft  and  arranged  to  rotate  close  to  the 
poles  of  strong  permanent  magnets.  Eddy  currents 
are  set  up  which  hold  back  the  disc  with  a  force  pro- 
portional to  the  speed,  and  the  result  is  that  the 
speed  is  made  proportional  to  the  driving  torque  of 
the  armature.  In  induction  watthour  meters  the 
retarding  disc  serves  also  as  armature,  the  revolving 
field  setting  up  in  one  part  of  it  eddy  currents  which 
cause  it  to  move,  and  the  stationary  magnets  setting 
up  in  another  place  currents  which  retard  it. 

Curve  Tracing  Meters. — ^Many  meters  are  in 
service  which  make  graphical  records,  or  charts. 
The  curve  drawn  by  a  recording  voltmeter,  for  in- 
stance, tells  the  voltage  at  every  instant  during  a 


ELECTRIC    METERS  85 

period  of  twenty-four  hours.  New  sheets  are  in- 
serted daily  and  thus  continuous  record  is  kept, 
which  at  any  future  time  may  be  called  upon  for 
information  regarding  pressure  fluctuations,  short 
circuits,  etc.  Station  operators  who  fall  asleep  on 
the  ^'graveyard  watch''  sometimes  are  thus  betrayed 
by  a  record  of  voltage  too  high  or  too  low  during  half 
an  hour. 

Such  an  instrument  includes  a  meter  with  a  pen 
mounted  on  its  pointer,  and  a  clock  for  moving  a 
piece  of  paper  uniformly  past  the  point  of  the  pen. 
The  mechanism  of  the  meter  may  be  similar  to  that 
of  an  ordinary  electrodynamic  voltmeter  or  watt- 
meter, but  with  sufficient  turns  of  wire  to  give  strong 
forces  to  overcome  pen  friction,  etc.  Other  record- 
ing instruments  make  use  of  relays  so  that  the  pen 
is  moved  by  electromagnets  operating  when  the 
metering  mechanism  closes  certain  contacts. 


XIII 
LAMPS   AND  ILLUMINATION 


Commercial  and  home  portrait  pho- 
tographers use  gas  filled  Mazdas  in 
deep  reflectors  to  illuminate  their 
subjects. 


LL  sorts  of  lamps  are 
used  for  producing  ar- 
tificial light,  but  in  this 
country  the  incandes- 
cent lamp  is  used  to  a 
greater  extent  than  all 
others  combined.  Until 
recent  years  the  car- 
bon filament  lamp  of 
this  type  was  the 
standard,  but  the  su- 
perior economy  of 
metallic  filaments  has 
caused  the  carbon  lamp 
to  be  practically  displaced  by  the  tungsten  lamp. 
The  latter  gives  approximately  three  times  as  much 
light  as  the  former  for  the  same  power  consumption. 
In  lamps  of  medium  and  large  size  it  is  found 
that  the  efficiency  is  increased  by  filling  the  bulb 
with  an  inert  gas,  such  as  nitrogen.  The  ordinary 
tungsten  lamp  has  the  air  removed,  the  filament 
being  allowed  to  glow  in  an  almost  perfect  vacuum. 
The  gas  permits  the  filament  to  be  heated  to  a  higher 
temperature.  This  means  that  a  larger  proportion  of 
the  energy  expended  in  heating  the  wire  is  radiated 
off  in  light  waves.  For  instance,  the  vacuum  lamp 
in  the  100-watt  size  gives  only  80%  as  much  light 
as  the  gas  filled  100-watt  lamp. 


86 


LAMPS    AND    ILLUMINATION 


87 


Lamps  carrying  large  currents  have  better  effi- 
ciencies than  those  of  smaller  amperage.  The  200- 
watt  220-volt  Mazda  has  the  same  efficiency  as  the 
100- watt  110-volt  lamp,  and  only  86%  as  high  effi- 
ciency as  the  200-watt  110-volt  lamp. 


Local  lighting  gives  the  largest  proportion  of  the  total  light  of  the  lamps 
at  the   point   of  use 


Candle  Power  and  Foot  Candle. — If  an  incandes- 
cent lamp,  hung  in  the  usual  position  with  base  up- 
ward, gives  off  in  a  horizontal  direction  as  much 
light  as  a  standard  candle,  it  may  be  called  a  "one 


3   O 


si 

ot'T 


Ho; 


LAMPS    AND    ILLUMINATION  89 

candle  power"  (one  c.p.)  lamp.  More  accurately  we 
say  that  its  "horizontal  candle  power"  is  one.  This 
point  must  be  emphasized,  for  the  amount  of  light 
sent  in  other  directions  is  not  the  same. 

If  a  lamp  is  located  at  the  center  of  a  globe  or 
sphere,  it  sends  light  to  nearly  every  point  on  the 
inner  surface;  but  different  amounts  to  different 
places.  The  average  candle  power  in  all  directions  is 
called  the  ''mean  spherical  candle  power,"  and  it  is 
usually  considerably  less  than  the  ''horizontal  candle 
power."  Ordinary  lamps  have  a  mean  spherical  c.p. 
equal  to  about  .8  of  the  horizontal  c.p. 

If  a  very  concentrated  one  c.p.  light  is  one  foot 
from  a  wall,  the  illumination  at  the  point  on  the  wall 
nearest  is  one  "foot  candle."  Every  other  part  of 
the  surface  is  less  brightly  lighted,  since  it  is  more 
than  a  foot  away.  However,  if  the  wall  were  warped 
so  that  a  considerable  part  of  it  was  exactly  one 
foot  from  the  light,  the  illumination  would  be  one 
foot  candle  all  over  that  part.  A  spot  on  a  wall  one 
foot  from  a  lamp  of  20  horizontal  c.p.  would  be  illum- 
inated with  an  intensity  of  20  foot  candles. 

For  various  purposes  different  intensities  of 
illumination  are  required.  In  the  operating  room  of 
a  hospital  the  illumination  on  the  "working  plane" 
(the  plane  level  with  the  table  top)  should  be  12  or 
more  foot  candles;  in  a  dining  room  2  foot  candles 
would  be  satisfactory.  Following  are  the  suggestions 
of  various  illuminating  engineers  for  a  few  cases : 


Auditorium 

1  to 

3 

Lavatory 

2  to     6 

Cigar  Store 

4  to 

6 

Library  (tables) 

3  to     4 

Coil  Winding 

4  to 

12 

Office 

4  to  10 

Department  Store 

4  to 

10 

Outdoor  Construction  .5  to     2 

Drafting  Room 

7  to 

12 

Proof  Reading 

4  to  12 

Drug  Store 

3  to 

8 

Residence-Cellar 

0.6 

Elevator 

1  to 

3 

Residence-Kitchen 

2.0 

Engine  Room 

3  to 

9 

Residence-Parlor 

1.5 

Garage 

3  to 

9 

Shoe  Store 

3  to     5 

Grocery 

3  to 

6 

Stairs  and  Halls 

.5  to     2 

Laundry 

3  to 

9 

Telephone  Exchange 

3  to     9 

Reflectors. — Reflectors  or  shades  are  used  with 
nearly  all  incandescent  lamps,  though  they  absorb 
much  of  the  light  and  therefore  are  far  less  than 


90 


APPLIED     ELECTRICITY 


100%  efficient.  A  surface  of  porcelain  over  steel, 
which  is  much  used  in  shop  and  factory  reflectors, 
absorbs  about  35%  of  the  light  that  falls  upon  it. 

There  are  two  good  reasons  for  the  use  of  bell- 
shaped  reflectors  for  interior  lighting:    (1)  they  put 


Indirect  lighting  in  the  new  and  beautiful   California  Theater,   San  Fran- 
cisco.    Note  the  individual   reflector  for  each   lamp. 


the  greater  part  of  the  light  where  it  is  wanted  and 
(2)  they  protect  the  eyes  by  making  it  impossible 
to  see  the  glaring  filament  unless  one  looks  in  an 
unusual  direction.     An  ordinary  bare  lamp  throws 


LAMPS    AND    ILLUMINATION  91 

very  little  light  downward  (past  the  tip),  and  so  is 
very  inefficient  if  hung  vertically  over  the  work  to  be 
lighted.  Furthermore  the  intense  light  which  enters 
the  eye  of  a  person  who  has  a  bare  lamp  within  his 
angle  of  vision  is  not  only  annoying  but  also  painful 
and  injurious. 

Indirect  Lighting. — In  some  cases  the  reflectors 
are  turned  upside  down  and  arranged  to  throw  all 
the  light  toward  the  ceiling.  Then  the  useful  light 
in  the  room  is  only  that  which  is  reflected  downward 
from  the  ceiling  or  from  special  white  surfaces  placed 
above  the  lamps.  This  is  known  as  "indirect"  or 
"totally  indirect"  lighting.  Note  that  the  reflectors 
are  completely  opaque. 

The  method  is  very  considerably  adopted  be- 
cause it  gives  freedom  from  eye  strain.  It  requires 
more  wattage  than  any  other  system,  and  tends  to 
decrease  rapidly  in  efficiency  on  account  of  the  col- 
lection of  dust.  Some  objection  is  made  to  indirect 
lighting  on  the  ground  that  shadows  are  largely  elim- 
inated, which  makes  it  difficult  to  see  the  details 
clearly.  It  has  been  claimed,  however,  that  with  the 
same  eye  fatigue  from  two  to  five  times  as  much 
drafting  and  similar  work  can  be  done  under  indirect 
lighting  as  with  any  other  artificial  light. 

A  modified  method,  known  as  "semi-indirect" 
lighting,  is  widely  used.  A  translucent  reflector  is 
put  under  the  lamp,  permitting  a  portion  of  the  light 
to  come  through  as  well  as  reflecting  much  .light  to 
the  ceiling.  This  is  fairly  efficient  and  has  many 
advantages,  but  is  open  to  some  of  the  objections 
to  both  the  ordinary  and  the  indirect  systems. 

If  reflectors  were  perfectly  efficient  and  walls 
and  ceilings  reflected  all  the  light  that  reached  them, 
absorbing  none,  the  total  light  flux  from  the  lamps 
would  reach  the  working  plane.  In  most  installations 
it  receives  only  from  20  to  60%  of  the  light  emitted 
by  the  lamps. 

By  multiplying  the  required  foot  candle  inten- 
sity in  any  room  by  the  area  of  the  working  plane 


92 


APPLIED     ELECTRICITY 


(which  equals  the  floor  area)  we  obtain  a  measure 
of  the  useful  light  necessary.  This  figure  has  to  be 
multiplied  by  a  factor  of  from  1.7  to  5  or  more  to 
find  the  total  light  the  lamps  must  give.    Below  are 


Warehouse  lighted  by  large  lamps  (300-watt  gas  filled)  at  a  total  expendi- 
ture of  .15  watt  per  sq.  ft.  What  is  the  size  of  the  squares  into  which 
the  ceiling  is  divided  by  the  lighting  fixtures?  What  part  of  the  light  is 
wasted  if  the  illumination  averages  one  foot  candle  at  the  working  plane 


listed  some  approximate  factors  for  small  rooms  with 
light  ceilings: 

Reflector                                         IJght  Dark 

Walls  Walls 

Prismatic  glass  bowl 2.7  3.0 

steel    bowl    (deep) 2.6  3.0 

Light  opal   glass   3.4  3.7 

Totally   indirect   5.0  6.2 

Semi-indirect    4.3  5.3 

A  25-watt  Mazda  lamp  has  a  mean  spherical 
candle  power  of  17.7.  If  it  were  surrounded  by  a 
spherical  shell  one  foot  in  radius,  the  inner  surface 
of  the  shell,  being  one  foot  from  the  lamp,  would 
receive  an  average  illumination  of  17.7  foot  candles. 
As  there  are  12.57  sq.  ft.  of  surface,  the  total  light 


Ordinary  Tungsten 

Watts 

Spherical  c.p. 

Total  light 

15 

10.0 

125 

25 

17.7 

223 

40 

29.4 

369 

60 

45.8 

575 

100 

79.5 

997 

LAMPS     AND     ILLUMINATION  93 

emitted  by  the  lamp  may  be  figured  as  17.7  X  12.57 
=  223  units.  Similarly  a  500-watt  gas  filled  lamp 
(mean  spherical  c.p.  =  694)  produces  a  flux  of 
694  X  12.57  =  8720. 

Such  numbers  are  found  in  the  following  table 
for  the  most  common  lamps: 

Gas   Filled    (Mazda  C) 

Watts  Spherical  c.p.      Total  ligrht 

75  69  865 

100  100  1257 

150  163  2050 

200  232  2920 

300  385  4830 

What  size  lamp  should  be  used  in  the  six  indirect 
lighting  fixtures  in  a  reading  room  23  x  30  ft.  with 
light  ceiling  and  dark  walls?  Take  foot  candles  =3.5 
by  the  first  list ;  the  area  =:  690  sq.  ft.,  hence  the 
useful  light  =  3.5  X  690  =  2415  units.  The  total 
light  =:  6.2  times  this,  or  15,000  units,  which  re- 
quires 2500  units  of  light  from  each  of  the  six  fix- 
tures.   Hence,  select  200-watt  gas  filled  lamps. 

Similar  calculations  are  made  for  many  effective 
lighting  installations,  but  the  design  is  not  generally 
as  simple  as  this  example  might  suggest.  Consid- 
erations of  art,  utility,  and  the  plans  of  the  owners 
and  architect  complicate  the  situation,  so  that  much 
study  and  experience  are  required  to  develop  power 
to  plan  satisfactory  lighting  systems. 


XIV 
INDUCTION— TRANSFORMERS— INTERPOLES 

Induction  Coils. — If  a  coil  carrying  current  is 
thrust  into  another  coil,  an  electromotive  force  will 
be  induced  in  the  latter,  due  to  the  cutting  of  the 


b 


1 

) 

■ 

V 

"""* 

Fig.  23. — The  Induction  Coil  consists  of  two  independent  coils  with  the  same 
core.  Varying  current  in  the  Primary  (P)  induces  e.m.f.  in  the  sec- 
ondary   (S). 

wires  of  the  second  by  the  lines  of  force.  If  the 
"primary  coir'  (P,  Fig.  23)  has  an  iron  core,  it  will, 
of  course,  have  a  greater  flux  than  otherwise,  and 
so  produce  more  **interlinkages"  of  lines  of  force 
with  the  turns  of  wire  of  the  secondary,  S.  The 
voltage  set  up  is  proportional  to  the  number  of  inter- 


94 


INDUCTION— TRANSFORMERS— INTERPOLES       95 

linkages  (number  of  lines  X  number  of  secondary 
turns)  and  the  quickness  with  which  they  are  pro- 
duced—  one  volt  if  the  rate  is  100,000,000  per 
second.  Leaving  the  primary  standing  within  the 
secondary  induces  no  voltage,  and  the  needle  of  the 
voltmeter  stays  at  zero.  But  breaking  the  primary 
circuit  at  K  produces  the  same  effect  as  withdrawing 
coil  P,  destroying  the  interlinkages  and  moving  the 
voltmeter  pointer  in  the  negative  direction.  Closing 
K  sets  up  the  linkages  and  gives  a  positive  indication 
on  V.  Thus  we  obtain  an  alternating  current  in  S 
by  starting  and  stopping  the  primary  direct  current, 
but  we  get  no  effect  with  a  steady  primary  current 
when  the  coils  are  stationary. 

If  we  replace  K  by  a  telephone  transmitter,  any 
sound  near  it  will  cause  motion  of  the  diaphragm, 
with  consequent  variation  in  the  resistance  of  the  in- 
strument. When  the  primary  current  rises,  it  in- 
creases the  flux  and  sends  current  in  one  direction 
through  the  secondary  circuit;  when  it  decreases  it 
produces  secondary  current  in  the  opposite  direction. 
The  apparatus  (P  and  S)  thus  used  constitutes  the 
"induction  coir'  found  in  every  telephone  circuit. 
The  ordinary  induction  coil,  used  to  shock  people  for 
the  betterment  of  their  health  or  to  produce  sparks 
for  ignition  and  wireless  telegraphy,  consists  of  the 
primary  and  secondary  coils  and  some  apparatus  for 
suddenly  opening  and  closing  the  circuit  at  K. 

Transformers.  —  When  alternating  current  is 
supplied  to  the  primary  coil,  the  apparatus  becomes 
a  "transformer."  The  flux  produced  by  the  primary 
and  linking  with  the  secondary  is  directed  first  one 
way  and  then  the  other  as  the  supply  current  flows 
forward  and  backward.  Every  time  the  flux  comes 
to  a  maximum  and  commences  to  decrease,  the  sec- 
ondary voltage  stops  and  reverses.  Thus  is  obtained 
an  alternating  e.m.f .  of  the  same  "frequency"  (num- 
ber of  cycles  per  second)  as  the  primary  current. 

The  ratio  of  the  induced  voltage  to  the  pressure 
applied  to  the  transformer  is  almost  exactly  the  same 
as  the  ratio  of  secondary  to  primary  turns.    Under 


96 


APPLIED     ELECTRICITY 


operating  conditions  the  secondary  voltage  is  a  little 
lower  than  is  indicated  by  this  relation,  on  account  of 
drop  due  to  resistance  and  the  "leakage"  of  part  of 
the  flux  (some  lines  are  produced  by  one  coil  and  fail 
to  link  with  the  other). 

Iron  cores  are  used  in  induction  coils  and  trans- 
formers, the  latter  usually  having  a  complete  iron 
path  for  the  flux,  while  the  former  have  **open  cores" 
which  are  merely  straight  bars  of  iron.  Of  course 
the  laminated  construction  must  be  used  for  all  cores 


Fig.  24. — At  one  instant  the  current  and  magnetic  lines  of  the  power 
circuit  are  as  indicated.  When  they  reverse,  the  lines  which  reach  the 
telephone  wires  induce  a  voltage  there. 

carrying  an  alternating  flux,  to  prevent  undue  losses 
by  eddy  currents.  (See  illustration  of  transformer 
construction,  p.  52.) 

Whether  or  not  iron  cores  are  used,  the  chang- 
ing flux  due  to  varying  current  in  one  circuit  will  set 
up  an  alternating  e.m.f.  in  any  other  circuit  with 
which  the  lines  become  linked.  Thus  the  apparatus 
of  Fig.  23  will  give  evidence  of  a  small  effect  on  V, 
even  if  the  coils  are  separated  as  shown  and  contain 
only  an  air  core. 

This  "mutual  induction"  is  often  troublesome. 
Coils  near  together  on  a  telephone  switchboard  used 
to  affect  one  another  and  produce  "cross  talk"  until 
each  coil  was  surrounded  by  an  iron  case  which  kept 
the  flux  from  straying.   In  a  telephone  line  near  an 


INDUCTION— TRANSFORMERS— INTERPOLES       97 

a.c.  power  circuit  (Fig.  24)  alternating  currents  are 
induced  by  the  lines  of  force  which  link  with  the  tele- 
phone wires.  It  is  to  overcome  mutual  induction  that 
the  conductors  of  power  lines  and  telephone  systems 
are  crossed  over  each  other  or  "transposed"  at  inter- 
vals. 

Self  Induction. — Flux  set  up  by  a  coil  links  first 
of  all  with  the  wires  of  that  coil,  and  this  interlink- 
ing produces  inductive  effects  similar  in  principle  to 


In  this  induction  motor  the  alternating  current  in  the  primary  winding 
is  choked  down  to  a  safe  value  by  self-inductance.  Short  circuited  currents 
are  induced  in  the  rotor  by  what  resembles  transformer  action. 

those  of  mutual  induction.  When  K  is  opened  (Fig. 
23)  a  spark  appears  there,  more  evident  if  there 
is  an  iron  core  in  the  coil.  A  voltage  is  induced  by 
the  change  of  linkages,  and  this  may  be  far  higher 
than  the  battery  voltage.  Indeed,  one  may  obtain 
a  very  perceptible  shock  with  a  small  electromagnet 
(such  as  that  in  an  electric  door  bell)  and  a  single 
dry  cell. 

The  voltage  induced  by  the  cutting  down  of  bat- 
tery current  and  consequent  reduction  of  linkages 
is,  naturally,  so  directed  as  to  oppose  this  diminution 
of  current.  Thus  the  current  is  kept  flowing  through 
the  increasing  resistance  of  the  opening  key,  crossing 
the  air  gap  in  a  spark  or  arc.  The  faster  the  gap  is 
opened  the  greater  the  voltage  induced. 

Low  speed  gas  engines  sometimes  have  the 
''make  and  break"  spark  for  ignition.  A  pair  of  con- 
tacts inside  the  cylinder  are  caused  to  touch,  and 


INDUCTION— TRANSFORMERS— INTERPOLES      99 

current  flows  from  a  battery  through  a  coil  of  high 
"self  inductance"  (having  an  iron  core  and  many 
turns  of  wire) .  When  the  circuit  is  suddenly  broken, 
a  spark  jumps  across  the  break  and  ignites  the  ex- 
plosive charge.  A  similar  device  is  used  for  lighting 
gas  lamps  and  stoves. 

When  an  electromotive  force  is  applied  to  a  cir- 
cuit containing  self  induction,  the  current  grows  but 
slowly,  for  the  increasing  interlinkages  induce  a 
counter  voltage.  An  alternating  e.m.f .  sends  through 
an  inductive  coil  a  current  which  is  small  compared 
with  what  it  could  send  through  an  equal  non-induc- 
tive resistance,  because  the  voltage  begins  to  de- 
crease before  the  current  has  time  to  rise  much. 
Furthermore,  the  voltage  falls  to  zero  and  reverses 
some  time  before  the  current  does.  Such  a  current 
is  said  to  be  "lagging,''  and  in  these  cases  the  power 
factor  is  less  than  100%.  Small  induction  motors 
often  take  current  lagging  so  much  that  the  power 
factor  is  80%  or  less. 

Commutation. — Fig.  25  represents  a  four  pole 
d.c.  generator  at  the  instant  when  each  of  the  brushes 
touches  two  commutator  bars.  The  coil  including 
inductors  numbered  1  and  6  is  short  circuited  by 
brush  A  at  this  time.  Just  before  the  brush  touched 
the  left  hand  bar,  current  was  flowing  in  inductor 
No.  1  the  same  way  as  in  Nos.  2  and  3;  an  instant 
later,  when  brush  A  no  longer  touches  the  right 
hand  bar,  current  must  flow  the  opposite  way  in 
No.  1,  for  it  will  then  be  under  the  south  pole.  The 
current  in  the  coil  must  stop  and  reverse  in  the  time 
it  takes  a  commutator  segment  to  pass  across  the 
face  of  one  brush — possibly  1/500  sec.  in  an  ordinary 
machine. 

Self  induction  keeps  the  current  flowing  in  the 
coil  after  it  is  short  circuited,  thus  producing  heat 
and  making  trouble  at  the  face  of  the  brush.  To 
remedy  this  an  "interpole''  (IP,  Fig.  25)  may  be 
placed  between  the  main  poles.  The  flux  it  produces 
must  be  enough  to  overcome  the  m^gjietizing  effect 
of  armature  reaction  and  in  additiaij  induce*  iii^  the 


100 


APPLIED     ELECTRICITY 


short  circuited  coil  a  voltage  opposing  the  e.m.f.  of 
self  induction  and  assisting  the  starting  of  current 
in  the  new  direction.  The  interpole  winding  is  con- 
nected in  series  with  the  armature  and  hence  its 
strength  is  proportional  to  the  armature  current. 


Fig.  25. — Four  pole  d.c.  generator  armature.  The  heavy  radial  lines  num- 
bered 1,  2,  3,  etc.,  represent  the  inductors.  To  avoid  confusion  the  brushes 
are  drawn  inside  the  commutator. 


The  interpole  shown  should  have  south  polarity, 
to  prepare  the  inductors  for  the  south  pole  they  are 
about  to  reach.  Three  more  interpoles  would  be 
used,  one  in  each  gap  between  main  poles,  and  each 
of  the  same  polarity  as  the  main  pole  which  fol- 
lows it. 

On  motors  interpoles  are  much  used  also.  Here 
each  one  has  the  same  polarity  as  the  main  pole 
which  an  inductor  passes  before  it  reaches  the  inter- 
pole. 


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;>iQV    22  iii3fe 


LD  21-95m-7,'37 


YB   125'^'^ 


O^ 


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